You are on a deep dive to something great like a wreak and in the briefing your dive guide says don't go below 5 on your NDL. On the dive you see an awesome turtle and want a closer picture. You glance at your NDL and see you are at 8, "I have 8 minutes" you think and drop down 20 feet and snap the pic. Satisfied you begin your ascent checking your air and then your computer to find your NDL is at 0 and you are in deco!
This happens more times than it should and many dismiss it thinking they will just do the required stop and be ok. Problem with that is, do they have enough gas (the mixture in your tank) to complete the required stop? Lets go more into deco and how to handle it. Note, this is not a replacement for actual training, deco is a serious thing and should always be planned.
A common misconception with NDL is that it represents 99 minutes on your computer. Your computer will start a countdown from 99 and as you spend more time or go deep that number will decrease. Referring to a recreational dive planner we see at 35 feet you have 205 minutes of NDL time while at 120 feet you have 13. These times may decrease depending on your previous dives.
In simple terms NDL represents your bodies space to absorb nitrogen, at 0 you can absorb no more. When you are at 0, an ascent straight to the surface will cause the large amounts of nitrogen to try to exit your body due to the decrease in pressure. This could cause bubble formation leading to DCS (decompression sickness). Think of a soda bottle. If you shake it up and open the cap quickly, soda will fizz forcing its way out the bottle causing bubbles (and a mess).Open it slowly, managing the gas escape and you will not get the fizz.
You took the picture of that turtle and now you are in deco. The first thing you should do is start a slow ascent following your computers instructions then alert your buddy. This is why it is important to read the manual for and understand your computer and be close to your buddy. Some computers may have you do what's called deep stops, telling you pause your ascent at specific depths for calculated times. Others may have you go straight to 20, 15 or 10 feet. This creates the issue of not knowing how many stops you have to do.
Your computer may say a 1 minute stop at 75 feet and you think after that you're good. Then at 20 feet it says to stop for 5, then another 3 minutes at 10 feet. We know from the dive tables, if your NDL is exceeded by under 5 minutes an 8 minute decompression stop is mandatory. How much gas do you have, is it enough? You may think, "My buddy can share their gas." but you assume they have enough gas to share. Now you've created another problem, DCS or CESA (controlled emergency swimming ascent). Maybe both.
Tips for deco
Tip 1: Watch your NDL to avoid it. No less than 5 is a great rule.
Deco itself isn't the danger, it's the lack of gas to complete it. Decompression stops can happen on more than just on a deep dive. 5 dives a day for a week on a liveaboard can find you close to deco on a 30 foot dive, this is why keeping track of your NDl is important. Plan your dives with your buddy using your computer or a dive table. If you go deep, follow the rule of thirds, 1/3 of your gas supply is set aside for emergencies only. Example: if you have 3000psi you should have at least 1000psi left when you are back on the boat unless there is an emergency. A pony works even better because it's not just an emergency gas source, it's redundancy.
Get the proper training, not youtube. Buy that tec instructor friend you have a beer and learn the basics. It will not certify you to do deco dives but it will give you a better understanding of them.