In order to perform alchemy a "circle" has to be created (most have runes). It's normally drawn on something eg. Kimblee in his hands, Alex on his gauntlets or Mustang on his gloves but anyone that has gone before the gate of truth becomes a circle themselves so they clap in order to create a full "circle" (when the hands join the become a "full" circle) it was also stated by Izumi that her body is like the runes on a circle