B737-700 CAPT INADVERTENTLY SHUTS DOWN BOTH ENGS AFTER CLRED INTO POS AND HOLD AT TPA.

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NORMAL PUSHBACK AND TAXI OUT TO RWY 36R FOR DEP FROM TPA. TOLD TO HOLD SHORT RWY AND RECEIVED EXPECTED TKOF CLRNC. WX W OF FIELD MOVING E DURING HOLD SHORT. TWR TOLD US TO POS AND HOLD AND TO EXPECT A DIFFERENT HDG AND ALT UPON TKOF CLRNC. CAPT TAXIED ACFT ONTO RWY FOR POS AND HOLD. AT THE POINT WHERE THE CAPT NORMALLY STOPS THE ACFT; PRESSES THE BRAKE PEDALS AND SETS THE PARKING BRAKE; HE DID THE FIRST 2 STEPS OF THE 3-STEP PROCESS; BUT SUBSTITUTED INADVERTENTLY SHUTTING DOWN BOTH ENGS FOR THE CRITICAL THIRD STEP OF SETTING THE PARKING BRAKE. AT THIS POINT; EVERYTHING GOT VERY QUIET. CAPT FESSED UP TO TWR THAT WE WERE STUCK ON RWY AND UNABLE TO MOVE FOR AT LEAST A FEW MINS. WE RAN THE APPROPRIATE CHKLISTS; GOT THE APU ON LINE; BOTH ENGS STARTED AND TAXIED CLR OF THE RWY. TWR WAS EXCEPTIONALLY ACCOMMODATING DURING THIS ORDEAL. WE AGAIN RAN ALL CHKLISTS TO ASCERTAIN WE WERE GOOD TO GO TO FLY AND OUR FMC WAS UP TO SPD AS WE SEQUENCED IN FOR A NEW DEP ATTEMPT. TWR AGAIN WAS VERY HELPFUL AND THE ENSUING TKOF AND FLT WAS UNEVENTFUL. IN ESSENCE; CAPT INADVERTENTLY SHUT DOWN ENGS ON RWY AFTER RECEIVING TKOF AND FLT WAS UNEVENTFUL. IN ESSENCE; CAPT INADVERTENTLY SHUT DOWN ENGS ON RWY AFTER RECEIVING POS AND HOLD INSTRUCTIONS; BUT PRIOR TO RECEIVING ANY CLRNC FOR TKOF. TPA TWR WAS VERY PATIENT AND ACCOMMODATING DURING THE WHOLE ORDEAL.