If the aircraft is flown in VNAV (vertical flight path) and is at say 37000 and a way point altitude of 6000 feet is entered at some point down line then the aircraft will descend to reach 6000 feet point in flight idle descent PROVIDED that 6000 has been entered in to the MCP altitude window, the reverse (climb) is true, but unless some one makes a further amendment to the MCP target it won't climb or descend again.
As far as I am aware, the 777 will not climb/descend unless the MCP ALTSEL is pushed, changing the cruise alt in the FMC, even with a step climb/descent programmed and a higher/lower level set in MCP altitude window. The step climb/descents entered into the FMC are for the FMC to calculate performance and fuel burn. However, the aircraft will descend when the TOD point is reached, VNAV changes to VNAV DES mode and a lower level is set in MCP ALTSEL.
Please correct me if I'm wrong.