Drive your knees fast and keep your core locked.
Keep your hips low and steady, do not let them pop up as you drive your knees.
Mountain climbers put your core in charge of a moving plank, every fast knee drive tries to bounce your hips up or sag them down, and your trunk's job is to allow neither. The running rhythm makes it as much conditioning as core work, which is why it appears in circuits everywhere. Done honestly, with hips pinned low and level, it is a plank that fights back at speed.
Piking the hips shortens the distance your knees travel, it is your body's way of making the drill easier. Re-set your plank, slow the pace, and only speed up as fast as your hips stay down.
As fast as you can go with a flat, steady trunk. Speed is the goal, but it is earned, a quick controlled rhythm beats a frantic scramble with bouncing hips.
Spread your fingers and press through the whole hand, and check your shoulders are stacked directly over your wrists. If they still complain, elevate your hands on a bench, which reduces the load without changing the drill.
