[my_controls]

Hm, I don't think that's what it does. If I leave 300 for 200 earlier than the computed T/D point then it will do a cruise descent but only until it recaptures the computed path. If it does intercept the path prior to FL200 then it will leave the cruise descent and continue in VNAV PTH down to FL200. This is why you have to watch out sometimes you get clearance to descend early and you reach the path say, 2-3000 feet before the cleared level and the plane noses over to maintain the path. At best uncomfortable at worst you bust the level I suppose although it seems to me it captures the altitudes quite well, even at high ROD/ROC.

I'm aware of the wind, but this phenomenon is instant. Basically here's what happens. I'm at, say, FL300. I get cleared FL200 so I select it and push VNAV intervention initiating a cruise descent at this point in time. I then go down to the CDU and enter FL200 in the cruise page. Immediately when I execute this entry I can see the speed bug move to a lower speed. This to me is the mystery.

Maybe I'm not making myself very clear :-)

It's the actual aircraft I'm talking about, forgot to mention that. Software 10.7 in the FMC I think it was.