Very interesting answers, thank you guys, you just helped me to kill this doubt, because when driving Formula Vee and vintage touring cars in my country, I found a very pleasant thing to do the heel toe technique, but these cars are equipped with a H pattern gearbox and you have more "space" between the gears to do the heel toe. Having said that, I found a little nonsense have to blip the throttle in a shifter kart, because in my mind it "cancels" the advantage of a sequencial gearbox..
Another doubt: in NASCAR races in road courses, I noticed that drivers who brake with left foot always blip the throttle, while drivers who brake with the right foot always do the normal heel toe technique.. My question is, if you try to drive these cars with left foot breaking, but whitout blipping the throttle in the downshifts, you will not be able to downshift or you will breake the gearbox?
Here is a video of Max Papis demonstranting the two techniques:
Is not better to do the normal heel toe technique than have the "advantage" of left foot braking but having to "wait" the time to blip while you find the neutral between the gears?