I assume the "doing" was a typo. "Bob" doesn't actually read this message board, as far as I know, so you have to settle for me.
A hydraulic hinge works just like the hinge that closes the door from your house to the garage. Check this website: http://www.shinwonent.co.kr/main.html
In your application, you need to select a hinge with a closing force greater than the weight of the door, and that involves some math that my head cold won't permit me to do for you right now, but I'll bet if you tell the people at shinwonent how much your door weighs and how big it is, and what direction it swings, (I assume you are talking about a trap door for a tree house,) you should be able to obtain a correctly powered hinge.