I am building an electric bike!!! What does this mean?!?

I need to add a throttle to my bike motor to help control the speed, but I don't know what throttle to use. Most throttles I've been eye-ballin' say that their a hall-effect type, with stats like "supply: 12volt" and "return 4 volts". Will this reduces the power of my motor, even at full speed? I am using a 12V DC Motor, and a 12V battery (12ah)
 
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