Push ups are one of the oldest and simplest exercises. When done correctly and consistently they work to build the upper body and mid section.
The reverse push up is a great addition, as it adds another core workout. Traditional people think that performing crunches is the best for your core, but there are many types of exercises that help to strengthen and stabilize the core.
To correctly perform the reverse push up, first lower your body into the traditional push up position.( if your a beginner, then positioning on your knees is just fine) Then instead of pushing up, from the plank position, bend the knees and push backwards while still on your toes. The arms with become straight.
Watch the video to see a clear idea of the proper technique.
This is an advance move, but with time and persistence it will become a great exercise with quick results.