Well, like zujitsu said, the vertical momentum will still be great, but if you transfer into a roll before you completely land and absorb all the vertical impact, some of that impact will go into forward momentum of a roll.
Think through it; if you do a roll
after you completely land and finish a drop, obviously the roll's not doing much since the drop is over and you've taken the impact. However, if you roll
while you are landing and compress at the right timing, it's as though you've "
half-landed" and you're still taking some of the impact while you go into the roll, transfering that momentum forward, thus lessening the overall vertical impact.
That's not a great explanation, but it's the best I know how to phrase it!
As far as what to look for in transitioning, you need to work on timing. Timing, timing, timing. If you roll too late after a drop, no impact is taken away. It's as though you're doing two completely separate movements. A drop. Then a roll. If you try to do it too early, you'll probably end up messing up the roll because you haven't landed on the ground yet.
So, as you land, you'll begin to compress your knees. As you begin to compress, aim with your arms where you'll be starting the roll, and lean into the roll, beginning the whole process. Your knees will probably extend again a little bit, and then you go into the roll.
Practice practice practice!
*Note: rolls are probably more effective when there is a drop with some forward momentum, like you said; A straight drop, especially from a substantial height, it would be difficult to even go into a roll because of the amount of downward momentum.
