Use Wedges on Big Trees
If you have a tree that's more than 18 in. in diameter, go ahead and make your notch cut and begin the felling cut. Stop cutting as soon as you've penetrated far enough to pound wedges behind the bar. Leave the bar in the cut with the saw running, but lock the chain brake and tap in the wedges. Then finish the cut. Wedges will keep the saw from getting pinched in the cut if the tree leans back.
Make the Felling Cut
Score a line connecting the apex of the notch on both sides for a cutting guide. The back cut should be parallel and even with the apex of the notch. Then make the felling cut. The instant the tree begins leaning, pull the saw free, set the chain brake and walk away along one of your escape routes, keeping an eye on the tree so you can react if it doesn't fall the way you planned. Never take your eye off a falling tree.
A Lookout Might Save Your Life
You'll be a lot safer if you have a trusted assistant standing a few feet behind you watching the top of the tree for falling branches and letting you know when the tree starts to fall. Have your assistant tap you on the shoulder with a stick to alert you when it's time to vacate the area. If it's early in the cut and you get the tap, leave the saw and walk away immediately. That means a branch is falling. Near the end of the cut, a tap means the tree is beginning its descent.