Are you using an airplane radio for this? (Remember that 72mhz isn't legal for ground vehicles, though 2.4ghz airplane radios are a-ok).

If you plug the ESC into the non-spring-return throttle channel, the mid-point would be "off", up would be forward, and down would be reverse. That's why a non-sprung throttle stick is annoying with a reversing ESC, as quite often it's hard to get it smoothly into the "off" position without the vehicle crawling slightly forward or backward.

D.