[With_Two_Flakes]

Rule 7-1-3-b-2
The player on each side of and next to the snapper may lock legs with the snapper, but any other lineman must have both feet outside the outside foot of the player next to him when the ball is snapped.

This Rule applies to all plays that starts with a snap, therefore it includes any scrimmage kick plays (punts, FG, PAT kick).

Even though the Rule applies to running or passing plays, it is most likely that it will be on a scrimmage kick play that the offense will break this Rule. By locking legs before the snap, they get an advantage to stop the defense getting through the gaps to block the kick.
Certainly in my officiating career, I have only ever seen this foul on a scrimmage kick play and maybe only 3 or 4 times in 20+ years.

So Referees and Umpires should start looking for it before the ball is snapped, so they are ready to drop their flag when the ball is snapped. It is a live ball foul. Remember not to flag it if it is the snapper and the guy on either side of him, they are allowed to lock legs.

Some well drilled teams will (after the snap) retreat into a blocking position so that players have their legs interlocked. That is not a foul. That is why you must see whether the legs are interlocked before the snap. You can't flag something you see 1 or 2 seconds after the snap.

Hope this helps....