Korg prologue Owner's Manual page 57

If the parameter value does not change
In some cases, the parameter value may not change, even when you turn the
knob from left to right. This means that the knob mode is set to "Catch".
In this case, the actual value of the parameter you're editing (the value shown in
the display) will not change unless it matches the position of the knob. In Catch
mode, the knob position and the value will be linked only when the knob posi-
tion reaches the actual value. This prevents an unnatural change in sound that is
caused when a value suddenly changes.
Let's say that you've turned the knob to edit a certain parameter, and
that the knob is in the position shown.
The actual value of the parameter is at the position shown by the tri-
angle.
The parameter value will not change until the knob position reaches
the triangle mark.
Once the knob has reached the position corresponding to the actual
value, the parameter value and the knob position will be linked, and
the value will change as you turn the knob.
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