how do i change velocity on zap amp?

[danny_noobs]

i cant find out how u do it can some1 please be a little detailed about this please....(but then beggers cant be chooses..)

 

[Rosie]

There are two knobs on the back, the one on the bottom controls velocity, twiddle it when the gun's over a chrono, sorry couldn't find out which way is up or down.