You sit at the bench press device, back slightly tilted to the back. The bar should be about 20 cm in front of you. Then you push the bar and take it back again, as you would with a bench press.

In this position you strain your chest muscles a lot less, which is nice if you want to train, but your chest hasn't recovered yet.

Here's a link to a girl on a machine specialized for this exercise, to give a better description than my failing words above.

http://www.schnell-online.de/db_imgs/products/img/t-80400.jpg