case #1: cruise descent, well explained in the FCOM.
case 2: active mode of the FMC is descent yet you will not descent through your MCP alt. Once you get VNAV ALT, go back to your VNAV Cruise page, re-enter the new alt and execute; voila! new descent path calculated and FMA back to VNAV PATH with a new TOD (unless you are beyond an achievable descent point).
Remember, if you are not happy with what the box is giving you, revert to basic modes to stay on flight path YOU want.