There are many types of push-ups out there, some easy and some not so easy.

When doing this exercise, start in the normal push up position then, move your right hand forward of your head and your left hand diagonally opposite.

Lower your body and when you push yourself up, do so in an explosive manner so that both hands leave the ground and whilst in mid air quickly swap your hand positions.  When you have landed your left hand will now be forward of your head and your right hand diagonally opposite.

This is a very difficult exercise so it is quite normal to be able to do less of these push ups than normal ones.

This form of activity is called Plyometric training and what this does is works the nervous system harder and also increase the speed or force of the muscular contractions.

If your shoulders are not up to this form of training then you can just change hand positions one at a time so that you always have one hand on the ground at all times.  It can also be done on the knees for those who have difficulty doing full push ups.