when you call animate(..) your object's mesh is updated according to given index. calculation is done via interpolating keyframes. that is every vertex of resulting mesh is an interpolation of corresponding two vertices in two keyframes.
for example if you have 3 keyframes for a sequence, and sequence number is 1:
Object3D.animate(0.25f, 1);
the resulting mesh will be interpolation of keyframe 0 and 1.
you should calculate index depending on your application and desired animation speed.
Object3D.animate(float, int)
this animates the model with given sequence number. (each 5 animation of your model is a separate sequence)
Object3D.animate(float)
this behaves like all sequences are merged into one