The following advice on making and using animations in Second Life is from Bloodsong Termagant.
If you have trouble with your avatar falling from great heights (hard fall) and getting up again, with the default animations trying to play.... You need to adjust the Franimation auto-stop time higher. I have found that 3.0 seconds works well.
It is a routine in Franimation that tries to circumvent SL's propensity to play the standing up and brushing off animation over top of your overriding animation, and then getting stuck in it. Basically, what you are telling the script to do is allow your hard fall animation to play longer before being cancelled by the script. (It must be cancelled by the script, or it may get stuck.)
Connect to the beta grid ( aditi ) to test animations. You can upload and tweak as many times as you want without spending any "real" lindens. Trying the animation in-world is the only real way to be sure it is working properly.
If you move any part of the avatar between the first and second frames in the animation, that part will be keyframed by the animation. If you do NOT move limb from frame one, there will be no keyframes, and the limb will revert to priority zero.
You do NOT have to leave frame one as the default T pose. Anything that moves between frame 1 and 2 will be animated by your pose/animation. Anything that does not change from frame 1 to 2 will be un-animated or 'unlocked.' Keep in mind that unanimated/unlocked does NOT mean immobile. In fact, it means the opposite. The underlying default animations will play on those unanimated limbs.
You may want to leave your head unlocked so the head of your creature moves naturally, turning to look at things like the default avatar head does. If your creature has any resemblence to hominids at all, you might find this handy for making your animation job easier.
For example, I made a pre-jump animation for Tinies, then tried to animate a jump -- hips up, arms swinging forward, etc. But I found I could actually just lock the extraneous limbs, abdomen, and chest for the jump animation, and let the default take over for the upper arms and legs, hips, and head. So with about zero work, I have a detailed jumping animation. The same is true for hovering.
In my standing and sitting animations, I left the shoulders unlocked, so the underlying arm movements of the defaults would give my Tinies some natural slight movements in their otherwise static poses. For flying, I unlocked the hips, and let the underlying default take care of banking and levelling out.
Example : Let's say you wanted to make a T-Pose, for use in a pose stand. If you leave the avatar in the deafult T-Pose in frame one, then make frame two the same pose.... When you bring it into SL and play it, apparently nothing will happen. You will have created a completely priority 0 animation. If, instead, you left the avatar in the T-Pose in frame two, but bent every limb and body part in frame ONE... then you would have a locked T-Pose in-world.
The big exception is the hip. No matter where you put the hip, or how you rotate it in frame one, that position/rotation of the hip will be considered the default zeroed position/rotation. This means you cannot lock the hip into its default position, nor height.
Example: Let's say you wanted to do a 'walking on air' AO, so in frame one you pulled the hip up 50 meters into the sky. Now you want the default animations to play, just at this new height, so you left blank keyframes in frame 2. Well, your avatar would not be transported up in the air. Instead, you need to leave the hip where it is in frame 1, then raise it in frame 2. This will lock it at that height. Unfortunately, it will also lock the hip's rotations, so when you try to sit or "sleep" or turn... your body will flail around while the hip is locked into place.