I would highly recommend that anybody who is flirting with mandatory decompression obtain and read Mark Powell's book, Deco for Divers.
What a dive computer is doing is taking the depth you are at and the length of time you are there, and calculating the amount of nitrogen that has been absorbed in a variety of "compartments". Each compartment has a half-time, which is the time it takes for it to absorb (or give up) half of the nitrogen it can contain. At any given moment, the computer is looking at all the compartments it's following, to see if the nitrogen in them has exceeded an amount which would permit a direct ascent to the surface at the ascent rate assumed in the model. Once this occurs, you have incurred a "deco obligation", in that you cannot proceed directly to the surface at that speed.
If you shallow up, you will begin to offgas. As you do so, you may lose enough nitrogen that all compartments will be below their nitrogen "ceiling", and at that point, the computer is satisfied that you can now proceed directly to the surface, and it will remove the deco warning. This is why, as you ascend on a multi-level dive, you gain NDL minutes.
Nitrogen absorption is always balanced by time in the water to offgas. As you approach or exceed your NDL times, the amount of time required increases and the importance of DOING that time also grows. Which is where the thing no one has discussed comes into play: Do you have enough gas to do the time? Technical divers exceed NDLs as a matter of routine -- but they do a tremendous amount of planning for adequate gas supply to do so, and for coping strategies for contingencies that might interfere with doing the required time. Most recreational divers don't do any of this. If you run up a deco obligation on a single Al80, do you have the gas to spend the required time in the water? What do you do if you have a freeflow and lose some of that gas, or can't use your own gas supply at all? Does your buddy have enough gas for BOTH of you to do the time?
One of the best instructors I have had the good fortune to work with, Joe Talavera, said something during a class that really struck me. Our class was divided into two teams, which meant Joe was going down and diving with one, and then coming back up and diving wth the second team. As he went in for the third dive, the captain of the boat asked him, "How are you for deco?" And Joe smiled, and looked at us, and said, "He's asking the wrong question. He should be asking me how I'm doing for gas. DCS generally doesn't kill you, but drowning does."
I guess I'm writing this because I think discussing how to put your recreational computer into deco and get out of it without bending the computer is a pretty dangerous discussion. Without the planning you need to do to be sure you have enough gas to do the deep portion of the dive AND enough shallow time to be safe from a nitrogen standpoint, you're taking some significant risks.
Everybody should also be aware that the algorithms in recreational computers are NOT designed to create a valid profile for staged decompression diving. Most recreational computers are not running that kind of decompression software (the X1 being an exception). They have simple fallbacks for exceeding NDLs, and the manual for the computer will warn you that the computer is NOT to be used to execute staged decompression diving.
Sometimes I just forget some things. It may have been me who used wrong definitions? I'm taking a little break on a family trip, $5 coffee and $6/hr internet so I'm not going to review now, but if you'd like to post accepted definitions, it might be a good idea - or skip it.
