The nozzel is engaged by the sector gear (via the tappet plate) before the sector gear starts compressing the spring (via the piston). This means the nozzel can be partially retracted without the spring being compressed.
Take a look at this animation:
http://upload.wikimedia.org/wikipedi...s/4/44/AEG.gif
Notice how the nozzel starts to retract before the piston.
A full cycle in semi shouldn't leave the spring partially compressed.