If the gap is too small or you saw tree completely, and if.
FELLING TREES Determine and develop a safe and efficient felling pattern from visual ob-servation. Take into account such factors as landing area, wind direction, natural lean of trees, tree species, and terrain. Prior to felling trees, identify hazards in the felling area such as widow.
The problem is that many novices - like I was about a decade ago - go into the woods on their own with no real appreciation of the dangers they will encounter.
The book then goes on about actually felling the tree, detailing various cutting methods and why you would use them. I will say at this point that the book only covers cutting trees from the ground.
Climbing a tree and making cuts while in the tree is a job best done by trained professionals/5.