WHEN TO SPEED UP
The best time to do power crunches is at the start of your ab workout, when the ab and oblique muscles have the most power. Just like with other power exercises, such as jump squats, if you are fatigued, the muscles will not be able to generate enough power to do the exercises as quickly and explosively as needed.
The diagonal muscle fibers of the obliques run from the lower ribs near your back to the front part of your hip bone. Separately, they work to flex your spine to the left and right, as during oblique crunches, when you bring your shoulder toward the same side hip.
They also initiate the twisting motion of your torso during twisting crunches, where your shoulder nears the opposite hip. When the left and right obliques work together, they help to flex the spine forward, such as during crunches and leg raises.