From a recent post on the current revival of disco rhythms, here's a straightforward model to understand the link between rhythm and dance, using the simplest of dances:
Consider marching in place to a count that goes: 1, 2, 3, 4; 1, 2, 3, 4; etc. One of your feet is striking the ground on each of those four main beats. Now insert a little "and" in between each of those four main beats: 1 and 2 and 3 and 4 and; 1 and 2 and 3 and 4 and; etc. These "and" spots are the offbeat, and correspond to raising your leg up, before striking it down on the ground on the main beat.
We're using 4 beats per measure because that's the standard. Giving them equal length makes each one of them a quarter note. When we squeeze in those "and" offbeats, that makes each note only half as long -- eighth notes. Squeezing in more, shorter offbeats produces sixteenth notes, but using this level of complexity is less common. (To sound it out: 1 and then it's 2 and then it's 3 and then it's 4 and then it's, etc.)
In addition to marching in place, consider some other simple rhythmic activities, and see which motions involve the main beat and which involve the offbeat.
Doing jumping jacks -- the main beat is when your feet hit the ground to your sides and your hands reach their highest point over your shoulders. The offbeat is when your feet retract to being right under you and hit the ground, and when your hands retract back to your sides. If you count out loud while doing them, you'll say the numbers when your limbs are most extended, while saying "and" each time they retract back.
Pumping your fist in the air -- your wind-up is completed with the offbeat, and the fullest extension is completed with the main beat. Counting out loud, you'll say "and" when your arm is fully wound up, and say the numbers when your fist punches as far as it's going to.
Nodding or bobbing your head -- you wind up your neck backward until the offbeat hits, then nod or bob it forward until the main beat hits. You'll say "and" when your head is furthest back, and say the numbers when it's furthest forward.
Squatting down and popping back up -- you reach the lowest depth and your knees are bent when the offbeat hits, and you are standing upright and your knees are straight when the main beat hits. You'll say "and" by the time you squat the furthest down, and say the numbers upon regaining upright posture.
Pelvic thrusting -- offbeat when your pelvis is furthest back, main beat when it's furthest forward. You'll say "and" when it's fully back, and say the numbers when it's fully forward. If you're moving your pelvis side-to-side to bump hips with someone next to you, the offbeat hits when your hips are furthest away from their target, and the main beat hits when they contact their target. You'll say "and" when they're furthest away, and say the numbers when they make contact.
You get the idea. Each of these activities consists of two motions -- a wind-up, and a delivery of force. Naturally, the fullest delivery of force should coincide with the strong main beats, while the less forceful wind-up should coincide with the weak offbeats.
During an actual instance of dancing, you probably won't be doing just one of these activities over and over, but these are some of the building blocks that can be assembled into a more varied dance, whether doing one at a time and changing them up, or doing several simultaneously.
To return to the broader topic of the inseparability of music and dance, the models above are not just learning aids or analogies. Musical rhythms are marked by offbeats and main beats precisely because music is so dependent upon dance -- it is designed to coordinate rhythmic bodily movement, whatever else it may or may not be doing in any particular song. (The converse dependency -- dance depending upon music -- is not controversial.)
Those bodily movements involve a wind-up motion and a delivery motion, so musicians must play two different kinds of beats. And since music is meant to dovetail with dance, the strength or weakness is matched between the musical rhythm and the dance motions -- with the weak kind of beat timed with the weak kind of motion, and the strong kind of beat timed with the strong kind of motion.
But why does dance music need to saliently mark the offbeats? Can't there just be rhythmic silence between the main beats -- 1, 2, 3, 4, with no "and"s in between -- and it's understood that the weak wind-up motion will happen some time during the silence, while the strong delivery will happen with the main beat? In other words: silence is weak, so why not pair silence with the weak motion of winding up?
Because although winding up is a weak motion, it does have a definite end-point, and it helps the body coordinate itself to the music if there's a clear marker of when the winding-up motion should stop, and the delivery motion should begin.
That's the problem that non-kinesthetic people have -- they draw out one motion for too long and start the next one late, or they cut one motion too short and start the next one early, so that their changing of motions is not in sync with the beat. Accenting the offbeat (in addition to the main beat) helps to keep both types of motion -- the weak wind-up and the strong delivery -- easily timed by the audience.
Try pumping your fist to a sparse count of 1, 2, 3, 4 -- you'll notice that your wind-up is not as drawn back, and you may not even be moving the upper part of your arm and shoulder, just moving your fist back and forth with your forearm only. Pretty crappy fist-pumping. But put an offbeat between each main beat, and fist-pump to a count of 1 and 2 and 3 and 4 and. Now you'll notice pulling your whole arm further back, feeling more coiled strength when the offbeat "and" hits, and then delivering a more forceful pump by the time the main beat hits.
For cerebral people, who are awkward in their own body, this may feel disturbing, like your body is being controlled by a musical puppeteer, or better yet like a wind-up toy. Each micro-motion of your body being marked with a rhythmic pulse -- it brings to mind the phrase "marching in lockstep," or movie scenes of an ancient ship's slaves rowing their oars while their supervisor bangs a drum to coordinate them.
But as usual with cerebrals, their rationalizing gets the better of them -- marking these main beats and offbeats helps the audience member to coordinate their body, both individually and collectively (as we already saw with doing the bump in pairs, or something similar in a larger group like a circle dance).
The regimented rhythm is not discouraging or shutting down something -- it is enabling and encouraging something. Without it, most people wouldn't be able to dance for shit. They'd embarrass themselves and turn off the opposite sex. With it, they've got enough of a kinesthetic aid to guide them through the motions effortlessly.
Moving your body in sync with the rhythm is not machinelike or robotic, it is allowing you to be fully human -- rather than just some disembodied mind. Nobody thinks of dancers, athletes, and other kinesthetic types as robots -- that would be the stereotype of cerebrals who are tone-deaf, have two left feet, fumble-fingers, and so on and so forth.
That's it for the overall discussion of why the offbeat must be marked in the rhythm of a dance song. So far, this marking has only taken the form of speech -- saying the numbers for main beats, and saying "and" for the offbeats. In an actual song, it's the percussion instruments that will take over this job, perhaps with some help from other instruments that are being played rhythmically rather than melodically.
In the next post, we'll get into the concrete details of the basic drum pattern for dancing, with a survey of examples from the restless warm-up phase of the 15-year cultural excitement cycle, when this kind of song emerges.