The tree was leaning slightly left, if you decifer the information in the pictures of the felling sequence the "tongue"comes out of the "groove"after the trees comitted to falling forwards no big deal it was the easiest spot to chip the debris.
A very basic/common felling tecnique in my local area,you have to do it to pass a level 2 felling ticket which covers your employers ass if you puk up.
once you know it and how to use it eliminates alot of hard work.