On my 2805 (small removable cover, ~.5"x3.5") it was just a plastic piece that pulled straight off. It wasn't hinged and was held on by plastic pieces inside it gripping the rca jack. It also came completely off, so you had to set it aside and hope you didn't lose it.
On my 3808ci (large attached door, ~2"x9")) it is hinged at the bottom. To open that you just push lightly at the bottom. It will then open fully on it's own. If you pull from the top it is very difficult to open and you'll likely break something.
HTH, those are the only two types I'm familiar with for the Denon recievers.
Jack