Ok, I actually went to the manual, this is a quote from the ent_animate section, read the last line.
If the scene contains bones animation, the instruction does not alter the frame and next_frame parameters, but modifies the entity's skeleton. Only bones affected by the given scene are set to their new orientation; bones not affected by this scene keep their old state. By executing this instruction several times for different animation scenes, bones animations are combined. Bones animation can also be combined with vertex animation.