FFS the door is not under pressure,It a a simple design that allows stones get trapped in under the door , if you havnt the experience to know that the back door is under pressure when tipped up but hasn't gone down and see the stone blocking it and try freeing the stone then your taking your life in your own hands without knowing it.
Please go and look at one of these trailers in operation, the door hardly moves in normal operations.
When as described a stone was jammed, then it is shear weight of the door and gravity that let the door drop striking the lad not pressure as you keep banging on about.