The snatch block is simple and works as @Old_Paint is intending. Technically, from a logging standpoint (and most of us are not loggers, myself included) it's mostly ineffective other than the initial stress placed on the tree, which as pointed out can promote barber chairing, though I should stress as @Old_Paint is using the setup so the bar is not pinched it does work:
Generally people pull on a tree to fall it a certain direction (which @Old_Paint is not doing and what I think you're questioning) and in that general case it does not work as the line goes slack as soon as the falling tree outruns the line, which is pretty quickly. The proper way to do this is use two lines, one on each side of the tree at an angle, so as the tree falls both lines guide it the correct direction. Using one line and not realizing how many tons a tree can weigh is the cause of losing control of a tree and it pulling a truck or tractor like a kid's toy as it falls "the wrong way."
If the tree is too big to pull over it can be wedged, jacked, and perhaps also pulled depending on the amount of back-lean, but this is better done with multiple lines and a crank tensioner. A bore cut or plunge cut can be used to help avoid a barber chair, and sometimes strapping or chaining around the trunk is also done depending on the amount of tension.
Also not-quite-yet-somewhat-related to this is the Palm Tree Hinge used to steer trees (youtube link #1) (youtube link #2).
DISCLAIMER: I'm not a logger. I have studied tree falling in-depth and succeeded in several extremely complicated falling cuts, each one being extensively planned and carefully executed, but that does not account for all the weird, unexpected, one-off situations that one should expect when falling trees.