When Marcus and Chantal are sitting at the table, the spoon in the aluminum pan moves from side to side, closer to her, then not so close.
Sherlock says catching a bullet seems dangerous, but is actually a very safe trick.
However, at least 12 people have died, performing the trick.
The guns used in the trick are real and there's always a danger to using guns. Some guns have been accidentally loaded, several have been deliberately loaded, one magician used to tamp down the gunpowder with his wand and the tip broke off and killed him when the gun was fired, and one magician performed the trick in front of an audience of cowboys, one of whom wanted to see him do it again in a local bar, with little warning and the cowboys own gun.
Holmes and Watson speculate on why the magic handbook was written anonymously when there is one simple reason and it is the same reason all magicians write guides and do TV shows anonymously, revealing the secrets of magic tricks results in suspension, or more usually, expulsion from the Magic Cirlce.
(Membership of the Magic Cirlce isn't mandatory for professional magicians, but not being a member severely damages their career prospects.)