Probably took absorbed of the impact before breaking.
I figured it took a hard hit, absorbing some energy. I didn't inspect the bottom of the notch after felling, to see if it had a dent from impact. During the fell, I was getting out of Dodge, so I don't know the details, with certainty.
In hindsight. A trucker's ratchet strap just below the bottom of the tongue would have helped it hold, not blowing out the side. Wider walls, narrower tongue would have been better.
I imagine that flaring outward to the rear does two things, possibly a double-edged sword.
First, it gives more fudge factor on the plunge cuts being dead-on square, to prevent binding while tipping, which might help break a brittle hinge, or prevent tipping altogether. A slightly flared bottom cut on the tongue might give a wedging area. I was banging wedges at 7'-8'. Not a big deal, we had to pull it with a rope, as it was solidly limb-locked. I wasn't forcing this tree over entirely with wedges. I looked at the down-sloping bottom of the groove, under the tongue, as another opportunity for a wedge, not as a replacement, necessarily, being mindful of not jacking the tree upward, breaking the hinge.
Second, it looks like the flared back of the tongue, coming down with force into the front of the groove could be used to try to trap butt of the log. In this case, it might be what caused the blow-out of the brittle sidewall. On a larger, green tree, that would allow a strong sidewall, possibly in conjunction with a trucker's ratchet strap for support just below the tongue, it might work. It would be just a slight outward flare. More dramatic flare would seem to create more wedging effect, blowing out the sidewall.
I think its obvious that most often, the three simple-cut felling method (two kerfs for the facecut, one cut for the back cut) suffices for the vast majority of trees cut, overall, and this is just a rare-use type of cut. I'd just rather practice when I get an easy opportunity with no/ low risk, and if it works out, I get a "gold-star" from the customer, and a talking point for the customer to relay to people, spreading my name.