I had a request for a manual roll drop from a client today, and I started to think about alternative ways to build one. I decided to do a search and found a few sources online for the traditional rigging techniques. I will make a distinction though - there are labeling issues. A roll drop tends to roll down - and only has enough weight in the bottom so it hangs / moves correctly. An olio drop, traditionally, has the roll at the bottom and its top is fixed. On olio drops you see the rigging mechanism (rope)on each side as the rope coils around the tube as it goes up and down.
Check out the links for more information:
How can I make an Olio Drop
Rigging a Roll Drop
Stage Rigging 101 This sight doesn't tell you much about rigging a drop, but it does have a nice overview of the subject.