OK I'll try a little, first consider this picture of a ship, the chain locker and the chain going out and to the bottom.
When putting the anchor chain to the bottom the chain comes UP from the locker, over a winch/brake machine and then over the side and eventually to the bottom Note that the distance from over the side (down) is considerably further than the distance UP from the locker and over the machine. SO, there is more chain pulling down, then there is chain being pulled up ... or more PULL DOWN then weight resisting. Just like a siphon and a bucket of water . . .
So when you are dropping the chain you must 'stay in control' or else gravity wins EVERY TIME. This is done in the hoisting machine which also has a BRAKE to control the out run of the chain. LOSE control ... gravity wins.
In order of what's in the youtube
1. Exactly what I just described. They are testing the equipment on a new build and the brake failed to hold .... chain runs away
2. Not sure ... I think the chain was running away and then tangled in the 'hawse pipe'
3. Brake fails and ALL chain runs away. RED SHOT is last one and when it reached the end the chain broke away from the ship and everything went to the bottom. EXPENSIVE if you can't recover the gear! And when the bitter end comes flying out of the chain locker it can whip around and cause damage/death ....
4. Chain run away again. Here the chain is not paying out to their satisfaction and you see a crew releasing the brake more and more so the chain will 'run' ... when it DOES run, the brake is SO loose, they can't tighten in time and the chain builds so much speed the brake actually catches fire from the heat generated ... (the proper way to pay out chain is for the ship to be backing down ... sort of 'pulling' chain off the ship and laying it on the bottom. When chain stops like this situation you just back a lil harder. You need to stay in control with the brake)
5. OOPS!