So 2 of the 3 would have been In The Back rven after this urgent rule revision ??
As to the difference, about 40cm or 50cm more reach and double or triple the force from extending your leg fully, over using your knee.
This rule has nothing to do with in the back. You're allowed to make forceful impact with an opponent's back when flying for a mark.
Can't believe how many people here think it's right to penalise Howe's and Reiwoldt's marks. Getting stops in your back is not an injury. I've got 30 year old scars on my back from that - no-one misses any playing time from a boot in the back from a player flying through the air. Toby Greene had one of his legs planted on the ground when he kicked out when marking in that game in Sydney - now that's a free kick. Bit of commonsense needed...