If you engage VNAV and are not on the path (or open the speed window), you will enter VNAV SPD as you would know. Above or near the idle descent path, you will get IDLE then HOLD. Below the path (or if you commence an early descent), you will get THR then HOLD as it gives you thrust to approximate a 1250fpm descent, and you can subsequently vary the thrust to modify the actual RoD. My FCOM describes this in the FMC Cruise section.