Many home owners cut for firewood, or to deal with trees that they do not want or are making a problem, people with larger properties need to deal with trees that have to come down. Dead trees, leaning trees and trees with rot in them are especially dangerous; you may not even know there is rot in some trees until it is too late. Many old trees with rot can split and kickback and they can kill you, even health trees can do this if they have a lot of lean.
Also not making a correct hinge and placing the back cut correctly can cause you to loose control of the tree.
In this video the guy is using the trigger method where you plunge cut and leave a small chunk of tree at the other side of the wedge cut on the tree, knowing how to do this may save your life.
Over head tangles in branches can bring a widow maker down upon your head.
I always look over a tree real well to see it's condition and where it wants to fall and where I can get it to fall. I use a helmet with a faceshild and earmuffs, and any safety gear I need. It is very important to have a escape rout on both sides of the tree at about 45 degrees away from your cut and away from the direction the tree is going to fall so when you see it move or hear a crack you can quickly get away. Know how to use wedges when needed and only use plastic or wood ones if your saw is in a cut.
I suggest that unless you are an expert (even if you are) check out the proper way to make a tree fall before you just cut one down, and some of the different methods you may not know about; even small trees can kill you. Also be careful when cutting the limbs on fallen trees, I have almost cut my leg by thinking the saw had stopped and relaxing my grip on it. I have cut down many trees most likly well over a thousand and I watch as many videos as I can to see what other people are doing. This method shown here is the best one for trees with rot or leaning trees, just be careful not to under cut your hinge.
Note; this guy is not using a faces hill, I would never run a chainsaw with out one.