There is nothing in the contract about being able to drop a trip if there is someone junior to you on reserve. That is incorrect. You can, however, drop trips anytime there is sufficient reserve coverage for the day(s) of the trip you want to drop. What is sufficient coverage? Only the computer and skeds folks know that answer, and I suspect that it changes daily.
You can also trade trips with open time, again assuming there is adequate reserve coverage for the day(s) affected. Reserve days can also be dropped and/or moved, and the contract DOES say that reserve day MOVE requests shall not be unduly denied. What does that mean, again who knows, but the bottom line is they won't let you move or drop if there is insufficient reserve coverage.
Dropping R days in my experience is harder than dropping trips, and as Dornier said, when there are plenty of folks in the seat it is much easier to drop stuff. If the seat is short on bodies, you can't even drop trips.
Holding a line (in my opinion) makes it easier to drop trips and build more days off.
FJ