The basic way you tune a cocker or sterling is the same
Changes in velocity are achived by swapping springs (hammer and valve), shimming in the case of older Stirlings, or using the velocity dialer (IVG) to change hammer spring tension. The main difference is the hammer spring is dialled from the rear on cockers, and from the front through the pump on later sterlings.
The inline regulator is used to set input pressure, and make a marker more consistent over chronograph. This should ideally be sweet spotted to match spring set. You do this by increasing input pressure until velocity starts to drop off, then back off input pressure to the point the highest fps reading was achieved.
Old markers were originally set up to run on CO2, and won't hit usable velocity on HP air without a secondary regulator. CO2 bottles run at around 650-700psi, compared to HP air at 850 psi. The higher pressure will hold the cup seal in the valve shut, fighting against spring tension and capping velocity
i don't know sterlings personally but by the looks of the picture its not a million miles from the Armson i have, if thats the case...
you should be able to wind the grey bolt our (to reveal more thread) the bolt would hit the Valve with more force and keep it open fractionally longer allowing more air to pass through the system at a greater speed. in tern launching the ball with more force.
I hope that made sense...
Does't show a valve spring behind poppet valve in that video
The input air pressure does hold valve closed to a degree, but it's operation also controled and adjusted by using different valve springs. He's correct where he states dialling velocity on regulator is a bad idea. If you cronograph past a markers sweet spot at say 600psi, and dial hammer spring to produce 300fps, the velocity will spike to well over 300fps when tank pressure drops to below 600psi. The lower tank input pressure allows the valve to stay open for longer. It would be held shut by air pressure bypassing valve spring over 600psi. This is why it's important to tune (sweet spot) the input pressure to suit a particular spring/valve combination on pumps and autocockers