This is an absolutely awesome upper body exercise. As you can see from the lateral movement, this pushup variation allows you to isolate one arm at a time. You are still balancing with both arms, but the lateral movement forces you to push with one arm only, taking the pushup exercise to a whole new level. You can stay focused on one arm at a time, as shown above, or move from one arm to the other. Also notice my hand position here. With normal pushups, my hands would be facing forward. Due to the lateral nature of the movement, I have opened them up. Yours don’t have to be as wide as mine are here, but I would suggest opening your hands more so than on a regular pushup.
I wanted to show the side view to make a few key points as far as technique goes. You’ll notice that even though my knees are on the ground, my body is still in one straight line. Make sure to keep your hips forward and core tight. This will result in that flat, straight line body position. You will also notice that my elbows do not flare out at a 90 degree angle from my torso. They are close to 45 degrees. Try to let your elbows come back while dropping down to the ground, as opposed to flaring straight out. This holds true for doing this exercise from the knees or from the feet.