A B737-300 ROLLED A FEW FT AFTER TOW BAR DISCONNECT BECAUSE THE PLT WAS NOT TOLD TO SET BRAKES. PUSHBACK TRAINING WAS IN PROGRESS.

Narrative

PRIOR TO PUSHBACK; RAMP AGENT CALLED ON INTERPHONE TO ADVISE THAT A NEW TUG DRIVER WAS BEING TRAINED ON HOW TO PUSH BACK ACFT. THE PERSON BEING TRAINED ON HOW TO PUSH BACK WAS VERY HESITANT ON THE RADIO AND HIS COM WAS BROKEN. AFTER PUSHING US OFF THE GATE HE STOPPED THE TUG. HOWEVER; HE NEVER SAID 'SET BRAKES.' I WAITED ABOUT 5 SECONDS AND THEN ASKED IF HE WANTED THE BRAKES SET; BUT NEVER RECEIVED A RESPONSE. I HAVE NEVER FLOWN INTO GRR AND WAS NOT SURE HOW THEY POSITIONED THE ACFT FOR PUSHBACK AND WAS NOT SURE IF THEY HAD COMPLETED THE PUSHBACK. I LIGHTLY DEPRESSED THE BRAKE PEDALS TO HOLD THE ACFT. THE TUG THEN SLIGHTLY PULLED ON THE ACFT. I WAS TRYING TO COM WITH THE TUG DRIER BUT TO NO AVAIL. I RELEASED THE BRAKE PEDALS; THINKING HE MIGHT PULL US FORWARD. THEN I NOTICED THE PLANE AND TUG WERE MOVING BACK TOWARDS THE GATE BUT AT A DIFFERENT RATE OF SPD. THE ACFT ROLLED APPROX 2-3 FT BEFORE I STOPPED THE PLANE AND SET THE BRAKES. FORTUNATELY; NO ONE WAS HURT; AND THERE WASN'T ANY EQUIP DAMAGE.