i have also reversed flushed the radiator while its in the car, but disconnetected the hoses.

i jam the hose in the bottom hose tight with the rad cap on, let it fill up, i block the top hose with my hand after its filled up a little to let the pressure build a little then let it flow out the top hose by releasing my hand.

I have seen a bit of scale and stuff come out this way and have seen improvements.

I also make sure i then rinse the core out bu removing the rad cap and flushing the raditor with its normal flow pattern.

by doing this it can dislodge some stuff by reverseuing the usual flow, and then fluching can rinse out loosened particles.