It has a little better output balance between the plate drive side and the cathode drive side.
Circuits like this have been used for years and years prior to the modern amp building community rediscovering it.
I use that mod in amps where the player is looking for every last ounce of clarity and the power tube driver stage is a split load or cathodyne type.
The HB I and HB II mods are in the same package and come with misc parts to split the common cathodes and tune the two preamp stages for less bass and other RC circuits to reduce bass flab and mud so the amp can be turned up a little louder with less roaring and unusable bass.
They work well with high output pickups and humbuckers.
Both HB mods come with mini-toggle switches so you can bypass them and go back to the stock sound.