Yamaha O2R Owner's Manual page 188

78 Dynamics
02R User's Guide
Knee sets the transition of the signal at the threshold. With a hard knee, the
transition between uncompressed and compressed signal is immediate.
With the softest knee, knee5, the transition starts before the signal reaches
the threshold and gradually ends above the threshold.
Release determines how soon the compressor returns to its normal gain
once the trigger signal level drops below the threshold. If the release time is
too short, the gain will recover too quickly causing level
pumping—noticeable gain fluctuations. If it is set too long, the compressor
may not have time to recover before the next high level signal appears, and
it will be compressed incorrectly.