52niner wrote on Oct 9^th, 2008 at 3:47am:

Thanks for the Tutorial, 1 more question, say im in flight and in the FMC I select rwy for landing then ATC pops on and says no land on this rwy. Now after I go into my legs page and make the nessecary descent changes and I change the rwy and hit execute. It won't follow my vnav profile. Now there is no discon in the FMC so wy would'nt my AC follow the profile I am telling it to ? I started my descenct ,changed my FL and hit FL button it would just descend , no restriction to the ALT i have in the CP. Thanks for any help

once you change runway and respective STAR to activate VNAV you press left LSK1 to copy the waypoint on scratchpad and hit left LSK1 again  this would copy the same waypoint on first location and exec would light up hit exec this way there would be route continuity from your planes position to the next waypoint.

For decending, after you change ALT you hit chase alt key. Once the altitude is reached autopoilot would hold alt until you assign new alt. So yeah you need to have constant input.