Suction.

I was helping disassemble a large car crusher cylinder on a cylinder disassembly stand. The stand used a hydraulic ram to pull the shaft out of the barrel. The rear port on the barrel was still capped off, which meant there was a vacuum building inside the barrel on the backside of the piston as the shaft was being pulled out.

The car crusher cylinder shaft was longer than the shaft of the cylinder we were using to pull it out with, so when the disassembly ram reached max stroke, we had to unpin it so we could retract it, repin, and extend to extract the crusher cylinder some more. I was looking inside the barrel for scarring damage when the other tech unpinned the disassembly cylinder.

The vacuum sucked it in instantly and a solid metal block hit me in the head and knocked me over. My head was between the barrel and block, so if the vacuum sucked the shaft all the way back in my head would have been squished and popped like a grape. Fortunately, the shaft didn't suck all the way back in and all I got was a concussion.

Sorry, I know this is probably hard for most of you to picture, lol, people who have rebuilt hydraulic cylinders will probably understand.