From official WT wiki:
For HE, overpressure damage is calculated when a shell fragment from an explosion hits something inside of the vehicle, proving that explosion got inside of it. This leads to high-yield explosives nearly always taking out a vehicle in a single hit, due to the shockwave being able to move around frontal armour and to hit the entirety of the crew that way.
Ok - but that being caused by a laughable 30mm AA HE shell at literally the top of a 45 ton tank? Eh…
And how can the IS2 HE shell overpressure my… ERA armor? Serious questions