Backpressure is simply pulling your opponent's arm toward your body and, therefore, away from his body. By pulling your arm closer to your body, you are able to exert more sidepressure on your opponent's arm. Conversely, if your opponent exerts more backpressure on your arm than you do on his, your arm will be pulled away from your body and you will lose a great deal of your potential sidepressure.

Sidepressure is the sideways force you exert on your opponent's arm when you take him down to the pin pad. You can exert much more sidepressure on your opponent by using both your arm and your body than you can using just your arm. The reason backpressure is so crucial is because the closer your arm is to your body, the more powerfully you are able to use both your arm and body to exert sidepressure on your opponent and win! If your opponent can pull your arm toward him, he has just taken your body out of the match and greatly increased his odds of winning! Look at the pictures below to get an idea of what is good and bad backpressure.

The puller on the left side of the picture (Jeff) is showing what bad backpressure looks like. Notice how his arm has been pulled to his opponent's side of the table. The puller on the right side (barT) is showing what good backpressure looks like. His arm is in tight to his body.

Here, both pullers are exerting good backpressure. On a side note, I have locked up with both of these wrestlers, and they both have STRONG backpressure!!