There are four types of radio technologies used in the U.S. for motorcycle intercoms. They are GMRS, FRS, FM, and Bluetooth (which may use other technologies to extend range). Frequency Modulation (FM) radio is similar to the FM radio you listen to, but for motorcycle intercoms a narrower frequency is used.

What hand signal do bikers give each other?

A fist with index and middle fingers extended together - The low wave if you're on a two cylinder bike. All four fingers extended - The low wave if you're on a four cylinder bike. A fist with the index and middle fingers extended but spread apart - A peace sign meaning "peace on the road."

What does it mean when a motorcyclist taps his helmet?

Tapping the top of the helmet is probably one of the most important signals that you need to know. Short answer, it's basically a warning to motorcyclist in the opposite direction that they should "slow down", there's speed trap or cop ahead.