FRC dives (Functional Residual Capacity), pretty much like diving after a passive exhale, where your lungs are at their "passive relaxed state". There are many benefits for training with this type of diving. First, it feels really really nice and relaxing. I know it sounds counter-intuitive but it's true, personally I enjoy my FRC dives more than my full-lungs dives. Part of it is because you are less buoyant, so you have very little effort to do on the way down and will start free-falling very early, which helps a lot with relaxation. One of the main advantages for training, is that in an FRC dive you mimic all that happens in your body during a deep dive, without having to go so deep: the pressure on your lungs and chest, the equalization etc... With FRC dives to 20-30m you will mimic all the sensations that you will have to deal with, much much deeper on full lungs. So this is a great way to get your body used to the pressure, to work on equalization etc. And you can repeat many FRC dives to 25m, whereas if you go to 60 or 70m on full lungs you will do one dive and then you're exhausted. You also get very strong MDR (dive reflex) with FRCs, so they're also great as warm-up dives before going for a deep dive. And then exhale dives are also a great tool for spearfishing of UW photography in shallow water, instead of over-weighting yourself. How to do it? Personally, I take a normal big last breath, and then let some air out slowly (so I can control exactly how much I exhale), usually to the passive point, then hold and start the dive. But be careful, this type of dive can be dangerous if you don't do it properly, so at the beginning it should always be done under supervision of an experienced instructor who will explain how to do it properly.