Never engaged while shifting gears, I’d only used the lockup switch once to see if it was working after the mods had been done. Had it engaged for about a minute or so around 15 or 20 minutes prior. I’ve rarely used the manual lockup since it was fitted years ago, as I find the trans automatic lockup did the job ok anyway.
Only find it useful to stop running away on long downhill runs. Maybe if I towed, I’d have used it more often.