2006 Johnson SD 30 HP 4 Stroke Outboards Service Manual, PN 5006592, Page 241Get this manual
POWER TRIM AND TILT OPERATION
Shock Absorber Circuit
Shock valve
Should the lower unit strike an underwater object while in motion, the piston will rise abruptly, creating sudden high impact pressure in the upper cylinder chamberThe shock valve will then open, allowing oil to flow into the area between the tilt ram piston and the free piston, thereby dampening (absorbing) the impact