this is interesting. i want to have a guess at what is happening here.

turbo surge happens when the compressor is making more air than the engine can consume. i don't know why we can't open the wastegate to slow down the turbine, and hence the compressor, at this point which should be possible if we have mappable boost and a suitable sized wastegate shouldn't it?

anyway, assuming we cannot slow the compressor down in this way we need another way. how about if the compressor gets up to a certain speed the air entering it will go mental and form a sort of a barrier in front of the inlet to prevent too much air being sucked in. that means that the engine will not be being force fed more air than it can handle which will keep the turbine speed down and reduce surge.

somehow the ported shroud makes the air go mental and not get sucked in

is that something like it?