Had this very thing last week in JV game. A53 stepped in front of A23 on a screen pass. #53 catches it and off he runs..flagged for illegal touching, 5 yards spot foul and loss of down. # 53 had big smile on his face :-)

One question on this one. My understanding has always been that B can push or pull to get to a loose ball so long as it is not pass interference. Similarly to get to the runner.

Situation #1: Sweep to the right. A20 is running down the field with A80 following closely behind. B50 in pursuit pushes A80 in the back to get to A20. I've always called and seen this called as legal.

Situation #2 (related to the OP): A5 is in the flat, behind the NZ. With the pass in flight, B10 pushes A5 in the back and intercepts the pass. I've seen this happen, and I've not seen it called.

Replace either case with holding, and I still have no foul. Indeed, I fail to see how B can ever be called for holding expect in cases where they pull an A player in an attempt to make a path for another B player (such as on punt of FG block attempts). In every case, B can push or pull to get to the runner or a loose ball.

Finally, as for clipping, I would call this. Since this is a block below the waist, I fail to see how it can be interpreted as an attempt to get to a loose ball or the runner. But it reminds me of a call we had some time ago.

Situation #3: A10 is running up the middle with A79 in front as a lead blocker. B5 dives at the feet of A79. We called an illegal block on B5 (much to the consternation of the crowd and coach).

The only case of contact below the knees I can see is if there is a loose ball at the feet of a player, and in an attempt to recover the loose ball a B player hits an A player. But that's the judgment call. I don't think B is obligated to avoid contact below the knees or from behind if it is an honest attempt to get to a loose ball or runner.

Oops. Going for the runner, contact above the waist.