Basically, the current in a circuit is dependent of the data manipulated: changing a value from 0 to 1 or 1 to zero generates a current to (dis)charge the gate

Basically, the current in a circuit is dependent of the data manipulated: changing a value from 0 to 1 or 1 to zero generates a current to (dis)charge the gate capacitances. Maxwell's equations state that a current generates an electromagnetic field, and this field is perceived by the antenna. The attacker is then seeing electromagnetic waves related to the data manipulated. By carefully comparing the waves with waves where the key is known, the attacker can then guess the key bit by bit.