yup that is the w2-g key, on high limit machines that have higher than 1200.00 and require handpay, turning that key would put the credits back on the machine (instead of removing) and when you were done you would get the w2-g for the entire amount won over 1200
ie
win 1300 regular reset key will remove creits and you would be paid and get w2
now win 1300 turn w2
win 1400 turn w-2
those 2700 stayed on machine and you didnt get cash
quit
you would get a singe w2-g form for the 2700, of course whether or not you lost it all