So, I've never really worked the strict "L takes everything" mechanic. On our crew we kind of hybrid it.
We give forward/backward to the LOS official who's key is the back. 90% of the time, that's the LJ. But if the LJ has trips, then the HL takes it.
If it's early in the play like within a second or two on a designed bubble screen, you just take it if it's coming at you. No reason that you can't handle this even if you have keys.
When there is a forward/backward decision later in the play, the person who's key is the back will take it (usually LJ).
If I'm the LJ and my key is the back and the play is going away from me, I like to run into the offensive backfield to let my partner know that I'm working backward. He can then make the call. I do this instead of punching right away because I've had times where the LJ punches and the HL doesn't see him due to traffic and calls it incomplete, or he hesitates so long trying to find the LJ that he's making a ruling really late and it looks very indecisive. I've been on both sides of this and feel this mechanic is my favorite way of handling this play.
To actually make the judgment, I go "point to point". I mentally drop a pin where the ball is released and where the ball is touched. I then connect the dots to see if it's forward or backward. In general, you can usually figure out the yard line as 1 yard in front or behind the QB's feet depending on which side he's throwing to.
