From elitefts.com -> q&a -> exercise index -> board presses:
"Board Press: This is a special max effort exercise designed to help strengthen the lockout of the bench press. It is also very effective in increasing tricep strength. This exercise is performed exactly the same as the bench press except you pause the barbell on a board that is placed on your chest. The board for this work out will be two 2 by 6 boards about 18 inches in length. make sure to pause the barbell on the boards before the ascent."
Depending on your weak points and the variety you want, people use between 1 and 6 boards to stress different parts of the lift.