..which boasts the concept of the little added 'pressure cup' (that clear plastic oval, in the pic, there..) that - in theory - helps add a 'pressure point', to the area..
This is still part of EMT doctrine. We are told to use something like a roll of tape, small rock, etc, any convenient small hard object that you can place under your wrap to apply pressure in a spot. I've used this method on a small but spurting scalp wound that gauze along wouldn't stop but a roll of tape, taped itself to the wound applied enough pressure to stop it in moments. It worked REALLY well in that case because you had the skull on the backside so it was really easy to pinch off the vessel.
But, in all, like Aer said: Contiguous and Sufficient Pressure, and these bandages Can help, especially if 'solo' (though not sure how you 'DIY-fix a blown-off butt-cheek'.. Not one I've 'BTDT' on..
The sad thing is, there a lot of injuries so traumatic that all you can do is hopefully increase your survival long enough to get to a surgeon, and if you can't get to a surgeon, you don't make it, even if the injury itself won't be fatal for DAYS.
Without a hospital, almost all gunshot wounds will be eventually fatal. Almost all compound fractures will be fatal or require ampuation (which will usually be fatal. Almost all avulsions will be fatal or result in amputation. 'Eventually' in this case could even be a matter of weeks.
The advances in surgery, particularly during WWII can really not be understated. These days, you can survive most trauma if you are able to get to the hospital alive. There was a semi local person who had a forklift fall on him from a bridge who lost over 40% of his body mass to trauma but lived. (Pelvis, both legs and one arm)