The bases of timing, at the very heart of animation, are found in a simple ball bounce. Timing is, graphically, the space from position 1 to position 2 - lots of space result in fast movement, little space in slow movement, OK, so everybody knows this... Now spacing is not the space from position 1 to position 2, which is timing :D, but the motion-path of a certain object. Richard Williams explains this with a ball example, and I think it's confusing that way, it's clearer in a hand movement, or a figure 8, where you can see the path clearly, and distinct from timing.
I love animating ball bounces, I use PAP to easily draw a bounce very fast, and then I draw another and another... it's nice and easy and a hellalottafun to draw them, and experiment with timing. A ball bounce can have a TON of personality, it can be lazy, it can be light, or heavy, or athletic, exuberant, it can be more organic-life-like or more object-like, it can have more or less squash and stretch, or only squash, or only stretch, or none of that. But what they all have in common is wider spacing as they leave the ground, narrower spacing around their highest position, and wider again as they return. Like this:
The red ball is the contact position, the most important position of them all :). "Animate contacts first" was the way Milt Kahl did it, and it looks like a really great way, since the contact position has 'no weight on it', and getting the weight right is 'of the essence', and it's easy to calculate weight by timing things right, starting with the 'weightless' position (as I already explained in another blog entry, there IS weight on this 'weightless' position, but the weight is suspended/floating because it has been projected upwards and/or forwards, and this is the... golden moment of suspension as the object is barely touching the ground, right before laying all the weight on the ground and another thrust will project the object's weight again). The contact position is a bit like an anticipation for the action that follows, it's the "ready?" position, and once you're "ready" that means you know what you're about to do, and you can "GO!";) It's easy to change weighting if you start with contacts: the contact will remain the same, and you only have to give it another start, referring to that same start-position. I strongly believe the best start position is a 'weightless' position, the contact.
The contact position is where you can easily decide the character of your bounce. If it's an object it should bounce back pretty fast, normally. If it waits for more than 3-4 frames it becomes a living thing (or a complex machinery) that needs to anticipate a jump. It becomes a jump, a conscious act of will, as opposed to a bounce, which is mechanic. Well, a living character can bounce like a ball, of course, but a ball - or any object - can never stay on the ground for more than 2-3 frames in between bounces, unless it's influenced by an external (or internal) force: like a water-balloon, say it's hard and elastic enough that it will bounce, the water inside will make a mess-of-a-bounce :)
At the other end of the bounce, its highest position can be more or less floaty. If it stays up in the air for a long time, it's very light (used as a cartoony bounce with very flexible characters), but then, if it's really THAT light :D it should also go up and down very slowly - think of an air-balloon. If it goes up quick and stays there for a long time... it's cartoony, it's an exaggeration, and I don't think there is any real earthly-object/matter that can generate such a bounce. A very light and elastic walk can barely touch the ground, go up very fast, stay up as much as possible, and come down slowly. A jump of a character that has a lot of weight, on the other hand, will not hover in the air but spend most of the time on the ground - fast going up, no hover, fast going down, lots of time spent on the ground.
The one thing that's impossible, but I've seen it done anyway :D, is - going up slowly and accelerating as you go up (and then maybe slowing down again at its other end). It's a piston movement, doesn't work for a ball; it works for all kinds of things that need an ease-in ease-out, but never in a jump or bounce.