With this mod installed, rogues will gain a passive class feature at level 12 called "Sneak Attack Master", which grants an extra sneak attack charge in much the same way other martial classes are granted extra attacks. If they can manage to make two attacks with advantage (or against threatened foes) in a single turn, they will now be able to trigger sneak attack damage on both.

It also removes the OncePerTurn cooldown from the sneak attack actions and reactions, which seems to be a relic of the "old" way sneak attacks were implemented in Early Access, presumably before sneak attack charges were introduced as a way of controlling how many sneak attacks could be performed. From testing, this restriction does not seem do anything other than ensure that each individual sneak attack action and reaction can only be used once; removing it does not allow three or more sneak attacks, for example, and so it appears to be pointless now.

Finally, it makes a small UI tweak, editing the "SneakAttack_Charge" action resource definition to make the number of sneak attack charges available appear above the hot bar like the other resources (actions, bonus actions, spell slots, superiority dice, etc). It doesn't (yet) change the icon used for these, so they show up as what appears to be the default: tiny five-pointed stars. Before acquiring the passive, rogues will have one of these; afterward, they will have two. Checking for these is a good way of ensuring that the mod is working as intended.

When I was testing, it did not seem to be necessary to respec in order for the passive to be added if you're already level 12 when you install this mod; however, if it doesn't show up and the mod seems to have installed correctly otherwise, that would be the first thing I would try.

Needless to say, this represents a substantial buff to max-level rogues over their capabilities according to 5e rules (and particularly to dual-wielding rogues, since they have an easy way of attacking twice in a turn). To be fair, rogues were somewhat nerfed in the vanilla game by not getting to use their reaction to make an additional sneak attack on an opponent's turn the way they should. This effectively returns what was taken away from them... with interest.

Like most mods, this should be installed using the BG3 Mod Manager. As far as I can tell, the "mod fixer" that many mods reference is obsolete as of Patch 2 and should not be necessary to get this working, although I have not installed it in a completely vanilla, modless setup to make sure.