The target: Your abs
Assume a push-up position with your arms completely straight. Position your hands slightly wider than your shoulders. Your body should form a straight line from your head to your ankles [A]. Lift your foot off the floor and raise your knee as close to your chest as you can without allowing your lower back to round. (Your lower back should remain naturally arched from start to finish.) Reverse the movement back to the starting position and repeat with your other leg. Alternate back and forth as quickly as you can. (It'll seem almost as if you're running or "climbing" up a mountain.)
If that's too hard: Perform the same movement, but place your hands on a bench, couch or other sturdy object instead of the floor. This reduces the amount of your bodyweight you have to support.