In the traditional Japanese martial arts the role of attacker and defender is well defined- uke and tori.
It is how many traditions practice learning waza and kata.
As a training partner, you attack me and I apply the technique, followed by changing, as I attack you and you apply the technique.
Many, if not most of the martial arts techniques are like this., but there are a few- perhaps a dozen or so out of the hundreds of waza that have tori “attacking” first.
Does one ever want to *move* before the attacker does?
In moving first, do you give something up by totally committing to a course of action?
If both training partners are in shizen- natural stance, ready to react, and one goes first committing to distance and timing, how can they change in the moment when the other person has not committed?