For the most part, the smith machine is one of worst pieces of equipment seen in gyms today (I will explain that at another time). BUT it can be used to improve your press-up.
The press-up is a great way to build a foundation of upper-body strength. It mainly builds up the arms, chest, shoulders and all the stabilizing muscles around the shoulders.
The problem is that a lot of people do not have the starting strength to begin with standard, full horizontal press-ups.
This is a different way to do a press-up. It is a good way for beginners to get stronger simply by lowering the bar on the smith machine a few inches at a time.
You can see how it is performed on this video at Bodybuilding.com. Give it a go, and soon you, too, will be able to do full press-ups.