Mario Kart 64 has a very simple premise, but there are a lot of secrets that experienced players know and use to their advantage. I will list several here (until I remember more!).
Starting boost - As the race is starting, when the traffic light switches from yellow to green (right when no light is showing), hit and hold down A and you will start accelerating as soon as the race starts.
Drifting - Get right behind an opponent and you will see white air coming out of the sides of your kart. You are drifting and will get a slide boost and can usually get past the person in front of you. Be careful they aren't holding down an item (banana peel, shell, etc.)
Power sliding - This is a surprisingly unknown boost that is very useful. When you're going around a turn, hold down R and instead of holding the joystick toward the side you're turning to, slide it back and forth. You will see white "E"'s coming out of the sides of your kart. If you're sliding the joystick back and forth properly, they will turn yellow and red for a nice boost.
Going against traffic on Toad's Turnpike - Win the gold cup in each of the grand prix and you will see an "EXTRA" menu. Go to EXTRA and play Toad's Turnpike. You will be going backwards (on any track in fact) but on this one, you will be going against traffic. Very trippy.
Hold items until you need them - If you pick up a lone red shell, green shell, blue shell, banana, or fake powerup, press and hold down Z and you will be holding it behind you. This allows you to pick up more powerups and be protected from someone firing a shell at you. Useful!
Koopa Troopa Beach shortcut - If you have a speed boost (mushroom or star), go for the shortcut! You will see a ramp that goes through to the other side of the beach. If you can make it, go for it.
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