Complex sentence: When the laser is in resonance with a cavity mode, intensity builds up in the cavity due to constructive interference and the laser is then turned off in order to allow the measurement of the exponentially decaying light intensity leaking from the cavity.
Split into simpler sentences: When the laser is in resonance with a cavity mode, intensity builds up in the cavity due to constructive interference. The laser is then turned off in order to allow the measurement of the exponentially decaying light intensity leaking from the cavity.