I know this frustration well.
It’s due to the ‘default’ tweener in Properties being set to “Bezier”.
The Bezier tweener will throw a spline curve onto your animation, which can be most useful in many, many situations… which makes it a great default setting for your tweeners. I still use Bezier as my default tweener, since it’s so useful.
For animations, we need to pick a main part of the figure to control the main “Translation” (movement away from zero) of the figure.
AniBlocks and most animated poses use the “Hip”, so I have adopted the same, but you can also use the main figure, or even a group in which your figure belongs.
Sorry for the slight babble - seemingly off the subject, but it’s very important to mainly use one only, and recognize early on which is used. If you use the Hip, for example, to move the character forward, and then use the main figure to move it later, you’ll be throwing off your whole animation. So pick one and stick with it. Later, you’ll find that this will lend well, because it leaves the other options freed up for other things, like adding an overall alteration to the entire animation, without having to tweak each and every keyframe of the Hip, etc.,
Okay, so I said that the Bezier can mess things up. Then I said that the bezier is very useful, so I keep that as my main default. Sound confusing? Probably. So here’s my brief explanation:
With Bezier set as the default tweaner all of my arm and leg, etc., (all, actually) keyframe changes are smoothed out and driven further by the curve of a bezier. This works differently the more keyframes are added, as the bezier will look at the next placement as well as the one after that and create an opposition to really drive a natural flow to things.
But when this bezier behavior is literally wrecking what we need, the quickest and easiest ‘fix’ is to select all of the Hip (in my example, using the hip as the translation joint) tweeners and change them to “Linear”.
Suddenly you’ll notice that the character no longer drops into the ground or other odd behavior. But it might also have the tendency to add a bit of stiffness to the movement, which is far easier to correct on a linear tweener by adding a few new keyframes, than to try and remove the bezier flow from bezier tweeners - if that makes any sense
Another technique to smooth out linear tweeners can be done by manipulating the In and Out sliders. This can also be used on bezier tweeners to change their behavior as well.
So the main reason I keep Bezier as my default is because I have found it to be the best main choice for most actions, and easy enough to switch for those joints that might go out of whack.
Another technique might be to go into Preferences and change the default tweener to linear, and then add a bezier if you want to use it in places. It all depends on how you like to work, and what it is that you’re doing.
You see, with Bezier set as the default, you have to remember that. If you change all of your hip tweeners from bezier to linear, but then add more keyframes, you’ll have to check the tweaners, because the default will remain bezier, if that’s how it’s set up.
Another real consideration against using bezier tweeners comes when working with speech and expressions, where “Mouth Open” and “Eyes Closed”, etc., can go backwards beyond 0.00 unexpectedly, which can really get you scratching your head going: “Why is the jaw closing into the nose, and eyes peeling back to reveal the entire eye sphere? Looks like some sort of Zombie!”
All in all, just know that if things look strange between the keyframes, check the tweeners and either work with their In and Out thresholds, or change them to a different tweener.
Oscillate is a repeating tweener. This one is amazingly handy! You can set a pose at the start, one at the end, and set it to oscillate three times to repeat the animation (like wings flapping) instead of having to set up more keyframes!
Have fun getting to know the various tweeners. They are there to make your animation experience as pain-free as possible, and work very well. They’re just very, very good at frustrating us before we realize what’s really going on. But once you know that it’s the tweeners doing it, it become fun and easy to get it right