Locked?
Although used and understood by experienced pistol shooters, I think the word "locked" like many expressions in shooting is the wrong word implying a physical action of "locking", that is not so, its where the wrist goes naturally when relaxed, hold and "aim" with no pistol, let the wrist drop into a relaxed position, the neutral as described in posts above, then adjust your pistol grip (if you can) into that position with the sight picture visible.
Most modern pistols will allow this with adjustments, the older ones won't, keeping the wrist held up. When I shot competitively I would cut the grip web as deep as possible on my pistol grips to achieve that, or as close as I could get.
Have Fun
Robin
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?