Hoppdoc,
It is a hydraulic recoil system call Coil-check I believe. It was installed by JD Jones of SSK when he built the gun. The system compresses appr. 2" in recoil. It is adjustable for the degree of pressure needed. Although you have to be careful as I learned. If you turn it to too much resistance, like I did, you can rupture the cylinder from very heavy recoiling rounds (that brutal recoiling 1040grn @ 1900+ fps). The main purpose isn't so much recoil reduction for the shooter, although it has this affect, it is to help prevent the stock from cracking in the wrist area. Cracked stockes is one reason JD recomends 900 gr loads @ 1850 fps. The action can take more. The shooter may be able to. But very heavy loads I beleive will sooner or later, crack the stock if your not carefull. My current heaviest load I will use now is the 900gr Woodleigh @ 1950 fps. It performes well and is tolerable to shoot for me @ I don't think the stock will crack with this load. That Beartoothe 1040grn @ 1900 was an unbelievable penetrator but I couldn't get good accuracy (5" @ 100 yrds)with it at this speed. When slowed to around 1600fps it shot much more accurate. I now cast my own 800gr 90% meplat flat nose below at 1500 fps as a great plinker and hunter. It is very accurate and shoots to the same POI as the 900gr Woodleighs @ 1950fps.
Long winded answer