Step 1: Place trapdoor on floor.
Step 2: Open trapdoor.
Step 3: Step into trapdoor (either crouched or standing).
Step 4: Close the trapdoor on your feet.
You may now note that if you were standing you cannot crouch, or if your were crouching you cannot stand up. In third person, you appear standing either way, but your hitbox and first person camera remains the same as it was when you closed the trapdoor. If you observe another player doing this, they appear correctly to you.