B757-200 CREW DSNDED BELOW THE ALT XING RESTR AT LAX.

Narrative

PRIOR TO REACHING CIVET INTXN; WE WERE CLRED TO THE SNRKL INTXN AND THE MITTS ONE ARR TO RWY 24R AT LAX. I WAS PNF; THEREFORE; I PERFORMED THE FMC WORK. I CONFIRMED THAT THE XING RESTRS WERE PROGRAMMED WHICH INCLUDED SNRKL AT OR ABOVE 12000 FT MSL. MINS LATER THE CAPT POINTED OUT THAT THERE WAS A RTE DISCONTINUITY BTWN THE ARR AND THE APCH. THE LENGTH OF TIME FOR THE DISCUSSION AND MY CORRECTION IN THE FMC WAS APPROX 10-15 SECONDS AT WHICH TIME I WAS 'HEADS DOWN.' WHEN I LOOKED UP; WE WERE DSNDING THROUGH 11700 FT MSL AND STILL ABOUT 1 MI FROM SNRKL INTXN. I GRABBED THE CTL YOKE TO RESIST THE AUTOPLT AT THE SAME TIME COMMENTING ON THE XING RESTR. WE CROSSED SNRKL AT 11500 FT MSL. I NOTICED THE CAPT WAS IN FLT CREWH MODE. HE THEN SELECTED VNAV MODE. I DO NOT REMEMBER WHEN THE CAPT SELECTED FLT CREWH SO OBVIOUSLY I NEED TO BE MORE AWARE. I ALSO COULD HAVE DISENGAGED THE AUTOPLT INSTEAD OF RESISTING THE FORCE. I THINK MY APPREHENSION OF DISENGAGING THE AUTOPLT WAS BECAUSE THE CAPT WAS PF.