Max is activated by hitting 123 at the same time. If you do it raw it has a little recovery, but if you do it after a normal hits or is blocked it cancels the recovery of that normal and lets you followup. So can reduce frame disadvantage on block and allows for new combo routes on hit. Its Tekken though so you have to see what buttons count as normals etc. You should check out that Geese youtube guide i posted earlier if you wanna learn more.
In max mode Geese also gets access to EX moves. He can't do EX moves when not in MAX. so whatever 'specials' he has all have EX versions in max.
And yeah its basically the system from KOF XIV. XIV is dope, look for the version with all the DLC included, and preferrably a sale, games been out for over a year now. Its hard(easier than XIII though) and the community isnt that big in USA but its a cool game.