how do you do power-slide turbo boosts?

To power slide in Mario Kart DS, you need to (drift) hold R (or L I can't recal which button it was) and start turning, while you're turning constantly press left and right on the D-pad. You'll begin to see red spark coming out of your back tires. Continue click left and right, and they will turn blue. Release the R button (or L, I Still can't remember which) And you will speed up a little bit. Please note though, that this does not wipe away the ink of a blooper on your screen. Also this is only recomended to use on corners.

Hope I helped

Ok when you go into a drift (power slide) press the control pad in the opposite direction of your turn and back until the sparks on your kart turn red. When you let go of the drift button you will boost. It will not remove ink. This can be used in curves one after another and even in straight roads if you have enough space this takes some practice but when you master it you can go faster than karts driving straight.

It's called Drifting you hold L to go left and keep pressing Left and Right, you do the same to go right but you hold are.