hey erm bout this downshifting thingy,i have a few comments.
at high speeds downshifting to a lower gear to get pickup requires practice and skill.u dun hafta shift it hastily and all to get into gear.as long as u can go pass the damn lori or another car its ok.
anyway downshifting while throttling up is a good skill indeed.its practised by many racers,especially drifters. this skill is important,especially when braking and dealing with corners,as the shifting of gears if not done properly will effect the weight distribution of the car.for drifters,it can either make the car stable into a corner or jus oversteer the car.it is a really important skill indeed,and requires a lot of practise doing it.downshifting while stopping down hard as u guys mentioned would make the skid and lose control.and also kill the gearbox and drivetrain.but then u can minimise the problem by getting into gear at the right rev.over rev will make ur car jump forward,will kill the gearbox,and under rev will do the same too,but worse effect to the car la.the car will somewhat jump backwards,and lose all power and momentum to go forward.
i had problems with my volvo clutchplate coz i let go of the clutch so fast everytime i go into a gear,and at the wrong rev.it kinda wear off hehe.so hafta pay rm1.5k for ori one.anyway,the cluthc plate will wear off firstly when it comes to problems like this,and you will have problems getting it into gear,and when it comes into gear,it kicks back to neutral.very damn dangerous.
anyway jus for extras bout the throttling into the right rev when goin into a gear really helps the car to stabilise.ive done doing some speedy action downhill cameron highlands with a trueno 92,it was okla,it was fun,and the method helps a lot.there was no unwanted wheel lock during braking and more traction required,so faster speed through corners lah. :( but then my car cant go very fast man...body turn like shit and tyre use wrong type huhu.suspensions also suited for old makcik going to the pasar....nuff bout it!
other than doing this method when picking up speed is to go to fourth gear la.but then go and overtake the car and change to gear 5 back again,right befre the cutoff point.as after the cutoff point,changing it into another gear will eventually lose all the momentum and also power.big nono.so study the peak torque of your car engine and applicate it la.
so bro,up to your judgement to do this method or not,and yea,one thing for sure,take care of the car and take care of urself.