What does player lock actually lock?

Any upgrades or mod movement you do after hitting the join button are not included since the state of your toons were locked at the moment of hitting the join button

BUT, in TW (unlike GAC), you can "leave" and re-"join" so that those upgrades you made can be included in the TW.

Can't do it in GAC (CG says) because there are waaaaay more players than guilds, therefore way more matchmaking/calculation that must be done than in TW matchmaking.

With new GAC there is zero MM computation so they can easily implement leave button to GAC

After that point players can't join.

Given match making is now on skill points not GP there is no reason to lock rosters at all other than moving mods between toons (so you can't just use the same mods repeatedly). There is no good reason upgrades are not allowed mid GAC any more.

There still needs to be a lock to participate though but it no longer needs to do much more than that.

Still, not an answer. What it does now is freezes your roster at the point you hit the button and tells the algo you are intending to participate.