One should always place the strap high enough on the tree that it is taught when you sit on the seat. You cannot fall out of the stand. I learned this the hard way. This is my post from another thread:
You want funny?
When I got my first hangon stand I put it on a tree about 4 feet off the ground in my backyard, just to get familiar with putting it on the tree and to see how comfy and whatnot. Well, it turns out that they give you a free safety harness, so I put that on to get familiar. So after sitting in the stand and talking to my Dad, who was standing there, I decided it was time to get down. I forgot I had the harness on and figured it would be easiest to just jump down. I jumped and right before my feet hit the gound the slack drew tight and all you hear is a big moan from me getting the wind knocked out. To add insult to injury I then swung around the tree and bashed off it several times. If you ever fall out of your treestand, your harness will save you from serious injury but it will still hurt a plenty.
Trust me!