I am thinking that it meant the vectors of the force of the bat and ball align from opposite directions at contact, with the resultant vector after collision being similar to the path of the force imparted by the bat. (Does that make sense?)Right on plane means ball and bat are traveling tangential to the same 2D surface(plane).
What I saw in her swing was an off-center contact, the collision of which resulted in the ball traveling in a third direction with a lot of spin.(assumed).
Anyhow, I went to a state school so I'm not trying to argue the point, just tell you where my thinking came from.