Yep, it's easy....
With rack lockouts the weight is transferred to the rack itself, rather than the person. If done correctly, it starts from a static postition and there is therefore no reversal strength. (this movement actually seems to have it's best carryover when a range of 3-4" are used) (Most guys can do much more in this movement than they can actually bench raw).
With board presses, the weight is transferred to the person, since the board is one your chest. The movement itself mimics a bench press and the groove should be kept as such. There is reversal strength involved as well. Make sure when doing boards to let the bar sink in a bit and then explode the bar off the board to lockout. (Most guys can do a fair amount of weight more than they can bench raw in this movement)
With floor presses, the load is transferred to the floor and the eccentric must be pretty slow or you'll fracture your forearms. Because of the way this movement works it turns it into a very a tough tricep pressing exercise. (Most guys actually do substancially less in this movement than they can bench press raw.)
Matt
