Here's what Grizz said in another thread:
Originally Posted by GrizzlyAdams
Thumbnail sketch...if your hammock is hanging from the ridgeline at 30 degrees, and the suspension line approaches the tree from the ridgeline at 20 degrees, then the total force on the ridgeline is half the body weight. Crank the tension up to make the angle on the suspension line to tree 15 degrees, and the force on the ridgeline is the body weight. Even tighter (if possible, e.g. using a trucker's hitch or something equally insane) to 10 degrees and it is twice the body weight. You get the picture...