Yamaha O2R Owner's Manual page 192

82 Dynamics
02R User's Guide
For ducking, this controls how soon the signal is ducked once the duck has
been triggered. With a fast attack time, the signal is ducked almost
immediately. With a slow attack time, ducking fades the signal. Too fast an
attack time may sound abrupt.
Hold sets how long the gate stays open or the ducking remains active once
the trigger signal has fallen below the threshold level.
Decay controls how fast the gate closes once the hold time has expired. A
longer decay time produces a more natural gating effect, allowing the
natural decay of an instrument to pass through.
For ducking, this determines how soon the ducker returns to its normal
gain after the hold time has expired.