This explanation is based on the technique of swinging a fly
MENDENITIS Definition – A term referring to the affliction of constantly manipulating your line voluntarily or involuntarily causing your fly to waggle about unnecessarily.
Please no mendenitis….
If you are ‘true quartering down’, Zero mend needed .
If you are casting at a 90 degree often times the best time to mend is after the fly is done drifting and comes under tension. I routinely see involuntary mends, and multiple ones at that, by anglers. These mends have zero purpose and generally are moving the fly throughout the water column.