The magician takes a silk scarf and starts rubbing it in the middle.
Believe it or not, the scarf pierces the mirror.
It can be seen very well on both sides.
You can push the scarf in or pull it out.
When the scarf is removed, the audience can see that there is not the smallest hole in the mirror which the spectators can examine from either side.

Silk Thru Mirror Stage with Fancy Design Work Locking