[couvillion]

You want the arm down position to have the direction of extension be pointed at the axle that the arm rotates about. You can then pre-charge the cylinder with out it moving. Then use a cam to break the linkage out of a straight line and it will extend rapidly gaining speed as the leverage on the arm increases.
It would be best if the piston wasn't all the way in when the arm is down. Hopefully geometry of the existing robot will favor you.