Like others have said, don't use terms like "set" or "planted" as that can come back and bit you in the but when you have to call a charge even with the defender still moving.
"He was moving forward at the time of contact."
"He didn't beat the player to the spot."
etc.
In fact, in general, I find that discussions/explanations with coaches are a lot easier when you stick to officiating terms and philosophies.