I was originally taught that the runners direction must change, but am now being told that is not true. Here are the relevant rules and case plays:
9-4-3k. No player or nonplayer shall: Grab the inside back or side collar of the shoulder pads or jersey of the runner and subsequently pull that opponent to the ground (Horse-collar).
OK, that says the BACK or SIDE of the collar must be grabbed, but says nothing about direction. But compare that to the second case play:
*9.4.3 SITUATION L: A1 is running in the open field and B1 grabs A1’s shoulder pad opening from behind and: (a) pulls A1 down abruptly backwards; (b) pulls A1 down to the ground from the side; (c) rides A1 for several yards before pulling A1 backwards to the ground; or (d) rides A1 for several yards before A1 falls forward. RULING: Illegal horse-collar foul in (a), (b) and (c), legal in (d).
In this one, the forward fall is not a foul because the defender didn't pull the runner right to the ground. The delay is what negated the horse collar, not the direction.
Here is the part that "contradicts" the rule, and leads to the Georgia interpretation:
*9.4.3 SITUATION N: A1 is running in the free blocking zone and (a) B1 grabs A1’s jersey collar opening from behind and pulls him down; (b) B1 grabs the front of A1’s jersey collar opening and pulls him down; (c) B1 grabs A1’s jersey at the top of the shoulder area and pulls him down. RULING: Illegal horse-collar foul in (a); legal in (b) and (c).
I don't think this case play really does contradict the rule. It says if you grab the front of the collar and pull him forward it is legal. Section a) seems to IMPLY that the pull must be backward. I guess the situation not covered is grabbing the side of the collar and pulling the runner forward to the ground.
Personally, I still think the Georgia interpretation is wrong, but I can't prove it in black and white rule or case language.