it's the same principle, it just requires greater body awareness, and more sophisticated timing.
start by trying diving pk rolls over an obstacle in a grassy area, and gradually increase the distance / amount of 'stretch out' in the roll itself. the biggest thing, is probably that you need to compress yourself into the roll much quicker, which requires alot of sensitivity with how your body is interacting with the ground beneath you. think sort of along the lines of how a slinky instantly begins re-compressing itself when it makes contact with the next stair. you want to begin tucking into the roll almost before you touch the ground, and you'll be pulling in with your knees and feet along a line more-or-less parallel with the ground, as opposed to bringing your upper body down to your legs. it's the difference between crouching down into a squat, and bringing your knees up to your chest when doing jumping squats. just remember to observe the other rules about pk rolls, especially tucking your head and looking behind you on the opposite side that you'll be rolling on, and also not forgetting to roll diagonally across your back, as opposed to down your spine.
start slow, and increase your speed and momentum gradually. if you practice fast, you'll learn slow; practice slowly, and you'll learn fast.
