What it strengthens:
Triceps, front of shoulders, chest, abs
These are going to be a more challenging push-up, but you can do it! Like most push-ups, these can be modified lots of different ways so they are able to provide a great challenge for your upper body for-ev-er.
Use an elevated surface, such as a counter top, sturdy table, or a high backed chair (that won't move). Place your hands shoulder width apart and walk your feet away so your body is at about a 45 degree angle. Pull in your stomach and bend your elbows straight back so they brush against your ribs. Lower your shoulders down to your hands as far as you can before pressing yourself back up to straight arms. A common mistake is letting your elbows wing out to the sides. If that's happening you may need to try an easier variation (like using a wall). Your hands should be in front of your armpits at the bottom, not by your neck or waist.
To make easier:
Do not bend as far down and/or use a higher surface or a wall.
To make harder:
Use a lower surface or do them from a kneeling position on the floor.