That safety guide sounds fairly good, though I used to be able to get into trouble even while more or less following it. Most of the trees were branch-locked to each other, so they weren't going to fall anyway. That is how I got into the practice of dropping them vertically two feet at a time by making parallel cuts across the trunk - never do that, it's dangerous as explained above. The weirdest problem I has was trying to drop a fair-sized tree (18" trunk at the point where I wanted to cut it) directly downhill, right close to my property boundary. After cutting the wedge, it gave signs of wanting to drop sideways onto the fence. With the equipment I had, I was fresh out of options, so I dismantled the fence, dropped the tree across the property line, cut the trunk into bite-sized bits, took them back onto my side, then put the fence back up. My main worry was that my neighbour would turn up while I was doing it, and kindly offer to drop the next tree for me, if I had that little control of the outcome.
