I submitted a video of me taking down an oak a couple years ago...posted it to get some feedback on how I was doing it. One of the best comments came from Burnham when he said he would like to see me doing more looking UP to see what was going on in the tree while I made my cuts. The info he took the time to write up was excellent and I saved it in my "Tree Stuff" directory. Here is about one tenth of what he took the time to pass on to me:
"When you are in the kerf, you can count on it that the saw will keep cutting . Don't just watch it work, lean back and look overhead frequently. More sawyers get hurt by stuff falling out of the tree on them than most any other cause. Watch out overhead. This is especially true when you are driving wedges. Don't just set up a regular pattern and drive away, hit a couple, look up and let the vibrations die out, then hit a few more."
I guarantee that most folks here cannot look up when they are stooped over low or on one knee making their felling cuts. When standing it is easier to lean back at the waist and look up for hazards or to see if the top is moving yet.
Also, standing on the balls of your feet as you cut is conducive to quick movement (survival) if something gets squirrelly.
I cut the notch at or just below waist level most times. (thanks, again, Mr. B)