B737-300
Flight Crew Last 90 Days 96; Flight Crew Type 96
Distraction; Confusion; Situational Awareness
Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Human Factors
Told to descend via the STAR.  Lowest altitude was 5;000 feet before DRAND waypoint; which has a 6;000 feet or above restriction.  Was given a speed assignment.  Went out of VNAV to open Speed Window and did not reselect VNAV. After passing DRAND intersection noticed we had descended below restriction. Approach Control did not mention the restriction and continued for visual approach and landing.  Pilot flying is new to the aircraft and did not notice the aircraft was out of VNAV PATH.
B737-300 First Officer reported the flight crew descended below a crossing restriction on approach when they lost track of the FMS mode.