Being able to uncrouch in a 1.5 area

When you go in a 1.5 block area with a trapdoor, if you try to close the trapdoor on yourself(like one would do to lay down), you uncrouch and can move normal speed as you would when you are normally walking. If you go out of the trapdoor you automatically crouch again.

I posted this yesturday but no responses so I'm re-posting