The birds mouth does not have more of a pinch. Make it 70 or 90 degrees. Same deal from the standpoint of hinge freedom. {I looked at Ekka's video again. I'd estimate 80º, then 90º, then 85º on the three palm tree notches. It sure looks to me like he got the max out of that particular wood and got the trees to drop and shortened the length of ground that the trees impacted.}
Humboldt’s with steep bottoms are good for dropping and helping and also with potential hang-ups.
A Humboldt usually utilizes less of the tree than a conventional face or traditional open face. Since the Humboldt has to be cut up higher on the stem than a conventional, it fails to bring home the 2/3rds of the stem in the area of the backcut. Up higher with the Humboldt backcut, this stays in the woods. Yes, all cuts can be placed low on the tree, it’s just that a conventional can usually be placed the lowest and get the most. Exception, trees being dropped downhill.
OK, so it doesn't make it into lumber, it still is fiber that makes it into building materials.
Side bar: the conventional cut also has its hinge pull wood in this fiber area, not affecting lumber potential. At my Father-in Laws mill, he specified conventional cuts for these two reasons. One of those guys from the depression and a family that had run a mill over a century that never wasted nothing.
It is a miss-statement that a Humboldt is a better cut because it doesn't waste wood at the stump, unless you're dealing with butt swell or downhill falling.
Most mills do prefer the Humboldt. Easier to work with particularly with head rig computer programs that cannot handle anything but a flat cut.