If it's a smooth animation, you could play only half of it, but if it's a single frame as I expect, I don't think so. If you want to get a popping effect where it turns x degrees, the number of degrees would be set in stone at run time, unless maybe you programmatically changed the key. But if you were going to do that, you could just program in the rotation, so that's silly. Only thing I can think to do is lessen the degrees of the rotation and play it twice, but I imagine you already knew you could do that. I don't believe you can play like 50% of a keyframe though or anything like that.