An analysis was made of the transformation of the electric area of ​​an electromagnetic pulse upon reflection from a moving mirror. Conditions are found under which reflection leads to an increase (in absolute value) of the electric area. For a dynamic resonator, one of the mirrors of which periodically oscillates, an expression is obtained for the threshold for generating pulses with an ever-increasing area. Nonlinear factors that limit the exponential growth of the pulse electric area are discussed.