In verification and response of what Quercus is saying I present a diagram. He says it's tough pulling wood apart, it's also tough compressing it to fail. The wood is strongest upon it's own axis.
In felling trees at ground level we are trying to get maximum strength, especially if we have hollow or decayed wood. Keeping in mind the model above you would agree then that a level hinge as displayed in the diagram below does 2 critical things.
1/ It carries the weight of the tree on the strongest axis.
2/ any force applied by wedging or even jacking is lifting the tree working the strongest axis.
There is less probability of failure this way, if the wind blows and it "sets back" then you have the force acting upon the fibres in the strongest way you can.
Now lets take a look at the angled back cut in the same situation.
What you'll see in the diagram is both the weight of the tree is now acting toward the weaker direction of the fibres and same when wedging. It could happen the fibre break and the tree slides down a bit pivots backward toward the cutter.
Also if the wind blows and you get sit back then it's also going to get force or leverage toward it's weaker axis.
There's also one other key point, it's difficult to get that cut right, level, even and the step height correct.
Now both Newguy18 and Quercus mention the use of this angled back cut for felling tops out of trees. The idea being, as he posted above, is that you get some barrier or ramp to prevent the butt coming back at you especially if it gets caught up in another tree. Now the step is supposed to do that anyway on a conventional back cut.
But hang on, he wants the hinge to break easier, the angled back cut is going to break sooner right?
Maybe, but why wouldn't you just have a thinner hinge from a conventional cut?
What about if it does get stuck with the traditional scarf (notch) and back cut if you do cut through the hinge then there's no ramp for the thing to slide off butt first (like a spear cut sort of).
So I present next diagram, and one that if you performed it on the ground felling it would be called a Humboldt. Perhaps have a slightly higher step too but this way you have a little better control and accuracy of your back cut.
Now I've felled a few tops and hollow trees and decayed trees etc and I can say that I dont do the angled thing. There's one thing I do with the angled thing though ... spear cuts, that's mainly on smaller stuff, mainly on branches not the verticle leader.
If there was a chance of the top getting caught in other vegetation whilst going over and getting stuck ... I'd go for the humboldt technique but seriously, haven't really had it happen and I dread it ever happening.