You need a big strong dog with plenty of weight, I had a 30 1/2" dog that weighed around the 95lb mark that specialised in this. You need the deer going flat out and it happens very quickly - the dog comes in from the side, strikes up at the top of the neck while also seeming to barge into the deer, deer goes over, and doesn't move again. Pre-ban, my current bitch (29" & 85lbs) took to using a very similar technique on roe and got it off to a tee - same technique she would keep turning them until they got into a straight line, would run level with their flank, wait her moment then strike