Pressing a switch triggers the spark gap, which releases the entire charge through the coil in 25 millionths of a second. This creates a huge magnetic field, which induces a current and then a magnetic field inside the coin, which in turn pushes back against the field outside. The repulsion force between these two fields crushes the metal, instantly taking a quarter down to the size of a dime. A large amount of energy discharged in a short amount of time usually entails an explosion, and in this case the copper coil is blown apart with a brilliant flash and a satisfying bang. And, yes, the report is sharper than any drum, proving that you really can hit something as hard with magnetic force as you can with a drumstick.