When it comes to blocks that happen on the "backside" of a play, there's no hard and fast rule officials follow. As a general rule, we call the things that happen at the "point of attack," meaning that they have an obvious, direct and immediate effect on the development on the play. Six-man football is difficult because plays can go all over the field and reverse course at any time, but we can't predict what may or may not happen on a play. If you have a sweep to the right and the end on the fair left side does a little jersey grabbin' on the backside, that's going to be let go. If they play shifts back to his direction and he's still holding on, it will begin to affect the play and you'll more than likely see a flag.
The backside stuff will be called regardless of the position of the ball if it is dangerous enough. Doesn't matter where the ball it, you can't just pick up the other kid and slam him down or take a cheap shot when he's clearly not part of the play and isn't even attempting to do anything.