A B737-800 CREW; DSNDING INTO LGA; UNDERSHOT A STAR XING RESTR.

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ON ARR TO LGA; WHILE DSNDING VIA THE KORRY 1 STAR WITH THE FO FLYING; I RETURNED FROM TALKING TO COMPANY AND SAW HE WAS HIGHER THAN 11000 FT XING RESTR AT FIX PRIOR TO KORRY AND WAS APPROX 1.5 NM TO KORRY AT 12000 FT; SO HE WAS GOING TO MISS THE 10000 FT RESTR AT KORRY. I SAID 'YOU ARE HIGH' AND EXTENDED THE SPD BRAKES. WE CROSSED KORRY AT APPROX 11200 FT. ZNY HANDED US OFF TO APCH CTL AND SAID NOTHING ABOUT OUR ALT. REMAINDER OF FLT WAS UNEVENTFUL. IT WAS SATURDAY NIGHT AND THERE WAS NO TFC AROUND US. ON THE GND; I DISCUSSED WHAT HAPPENED WITH MY FO. HE SAID HE GOT CONFUSED BY THE FMC DSCNT WITH THE SEVERAL XINGS AT OR ABOVE REQUIREMENTS BEFORE THE HARD ALT RESTR AT 11000 FT. HE MANUALLY OVERRODE THE AUTOPLT USING VERT SPD AND WENT HIGH ABOVE PATH WITHOUT REALIZING IT UNTIL IT WAS TOO LATE. I EMPHASIZED THE IMPORTANCE OF COMMUNICATING TO ME WHEN HE IS CONFUSED AND I AM OFF DOING ADMINISTRATIVE FUNCTIONS. IN THE FUTURE; I WILL NOT LEAVE MY MONITORING DUTIES PRIOR TO MEETING XING RESTRS.