Re: Catching on a fast break and coming quick off of one foot???
Sometimes dunking just isn't in the cards. I think the majority of folks need to have their footing right prior to takeoff in order to slam. That's why you'll see a lot of players implement a little bit of a shuffle prior to their takeoff steps to properly set up their dunk attempt.
Like everything else, I'd suggest staying quick on your toes while resisting a completely upright stance. I think the faster and quicker your feet, the less space and time you may need to get your body set for a dunk attempt. Picture the big men who have those large plodding steps in the open floor like Greg Monroe versus a guy like Russell Westbrook who has low, quick, choppy steps leading to the rack.
Personally, the more upright my body is, the more time it seems to take to stride. As such, there's a smaller window for takeoff as I must wait for one of my big steps to hit the floor. On the contrary, when I'm low and quick, utilizing quick-on-balls-of-my-feet steps, I seem to have a wider takeoff window to choose from. This is also true of your dribble. Higher dribbles means a reduced takeoff window (must wait for ball to bounce back). A lower dribble seems to offer more protection, more bounce, and a bigger window for attack on the rim.