I double clutch downshift my car when changing down gears at high rpm. Its supposed to be good for your box in that the engine speed matches the road speed hence the strain on the gearbox to adjust to the correct speed is alot less. I get within 200rpm of engine to road speed under hard braking.
If you have blown 2 gearboxes trying to do this then your just shit and i suggest you stop doing it before someone gets hurt. Could you please let me know when/if you are going to be attempting another heel toe monevouir so that i can stay a fair distance away to avoid gearbox parts being thrown my way.
Jeez i thought people knew about cars on these forums. Judging by some of the posts in this thread it looks like im wrong.