A large trunk is brought on to the stage; a few spectators are invited to examine it, top, bottom, sides and ends. When they are satisfied that it isn't rigged, an assistant gets in (someone who fills the trunk pretty nearly). The trunk is closed and padlocked; it is roped on all sides; the ropes are sealed where they cross each other; then the trunk is tilted on to one end, both so that a bag can be placed over it and to make it easier for the assistants to lift it on to a pair of trestles, when the bag can be tied off and sealed. The performer then says, "we will now make our assistant disappear"; he fires off a pistol (with an appropriate countdown), the bag and the ropes and the padlock are removed, and the trunk is empty!
The trunk has one of the short ends constructed to pivot on a rod through its center; it is held in place by a spring-loaded switch, small enough to be inconspicuous, but strong enough to hold the end firmly in place.
When the trunk is tilted on to its end, it is placed over a trapdoor in the stage. The imprisoned assistant presses the spring, the end pivots, he slides out through the trapdoor, and pushes the latter back into place — which action also causes the trunk end to rotate back into place and lock itself.
The assistants tip the trunk and lift it on to the trestles — not the spectators. This can be explained by some nonsense about liability if a spectator should wrench his back, or the like; but the real reasons are that the trunk must be tilted to the correct spot on the stage, and to cover the fact that the trunk has suddenly become much lighter when it's lifted on to the trestles. The assistants must indulge in a bit of acting here, to seem to be lifting a load.
The trunk need not be completely built of wood, etc.; the long sides can be made of glass. In this case, timing is exceedingly critical as the inside assistant can't move until the enveloping bag touches the floor.
It's possible to have another trunk on stage in which the assistant is found to be after having disappeared from the first one; but since the same rigmarole in sealing it up has to be gone through, including the tipping so that the assistant can get into it, there is a lot more time and opportunity for the audience to get an idea of what's happening. Besides — how many trapdoors can we xpect?
Care must be taken that the rope around the pivoting side is pretty well centered, so that the pivoting can occur smoothly. A little bit of suggestion to the spectators, an inconspicuous tidying-up by an assistant or the performer, will ensure this. Some visual aids won't come amiss, either; a stripe in the correct position on the end, or some rivets to indicate where the rope should go.
Naturally, a cover story has to be available to explain the assistant's eventual re-appearance.