I was recently getting back into Mario Kart Wii, and when I was playing noticed that from time to time, some blue speed lines would appear in front of the kart. Occasionally they would become very numerous and would give the cart a speed boost. What is this, and is there any way to cause it?
The blue lines mean you are drafting.
To do this, you need to follow behind another player. If you do it right, the lines will appear. If you can remain in that state for long enough, you'll gain a small boost.
Drafting is also used in the real world, though it works a bit differently than in Mario Kart.