how do they do it? i race someone on gba bowser castle, they have no protective item, i launch a red and (along the perfectly straight wall) they somehow maneuver and make the red crash into it. i've seen it on other courses too, what's the technique for it??

They either drive round a corner tight, so it hits it or drive towards the wall and then turn and it hits the wall.

You learn something new every day!

Red shells take wide turns, so to speak. If you have a good inside drift bike, you can quickly cut a corner and the red shell won't have time to react. There are other tricks to dodge them, for example, take GBA Bowser Castle 3 again: on the straight-aways that resemble a mesh grill over lava, you can jump over or dart behind the square-shaped cutouts to avoid reds.

I just cut corners

n64 Bowser Castle is superior for not getting hit by red shells, because there are so many corners you can cut=red shells hit the wall!

Yeah, but if there are no corners...

Just block your back with an item... its the easiest :)

except for bombs and fib's

just dogde around the walls