From your posts I believe you are talking about preloading the cylinder.
To do this you need to hold the cylinder partially extended and then fill the cylinder.
In an optimal configuration the piston will be extended ~.3-.4* the length of cylinder.
Most teams hold the piston in place using some kind of latch. Team 842 uses a standard gate latch and U bolt as part of their configuration while other teams use quick releases designed for water sports etc.
On the physics:
This does not increase the maximum force provided by the cylinder. That will always be given by system pressure*area.
Preloading the cylinder increases the system's energy output by maintaining that force over a longer distance. (Work is force integrated over distance).
Here are some useful links:
Regarding safe kickers:
http://www.chiefdelphi.com/forums/sh...ad.php?t=80679
Regarding the math/physics:
http://www.chiefdelphi.com/forums/sh...ad.php?t=81145
An Example (with photos!):
http://www.chiefdelphi.com/forums/sh...ght=842+kicker
I hope this helps and would be more than happy to answer any additional questions you might have.