One of the most common things yelled at basketball games (based on my non-scientific studies!) is “Over the back”. Everybody in the gym knows what this means: B1 is in front of A1 for a rebound, A1 gets the rebound.
But technically there is no ‘Over the Back’ foul or violation. The issue and rule is ‘displacement’: does A1 displace – move – B1 in trying to get the rebound.
Often A1 is taller than B1 and reaches in front of B1 or over the top of B1 to get the rebound, sometimes even touching B1. That doesn’t matter unless A1 displaces – moves – B1.
While on that topic, ‘boxing out’ has changed to also include the ‘displacement’ principle: If B1 pushes/moves A1 out of rebounding position (one of my childhood playing specialties), or A1 pushes/moves B1 under the basket to gain better rebounding position: FOUL. If they are both pushing against each other but no displacement, play on!