Primary muscles worked are the chest. Secondarily are the triceps and shoulders.

Adjust the bench as desired. The declined position will exercise your lower chest muscles, flat position mid overall, and inclined position the upper chest.
With your feet flat on the floor, adjust the pressing arms to the desired range of motion. If you lower the handles too much, it will put more stress on your shoulders.

Lift the handles to the top position without locking your elbows. Lower the weight with control but do not let the weight stack come together.