Before the hand moves, the elbow moves. Before the elbow moves, the shoulder moves. The shoulder moves because we breathe in before we breathe out an attack. Before we attack, we see an opening and the eye pupil dilates to to minimum emotional arousal and visual recognition. When the eye got darker, I entered.
This I can definitely believe. You can often see an attack faster by watching someone's face than by watching their actual hands...
Whether the sempai is randomly punching his junior in a way that's actually useful to the kohai or is just a childish way of saying 'ha ha I'm better than you' is something he'll have to figure out for himself, though. Or of course it could also be both (the motivations might be immature but you could still be able to learn from it).
I also like some of the comments above about learning to tell when people are BSing you! This is a valuable thing to learn to judge.