Many speed controls that are designed for racing will have reverse disabled by default. Many of the old brushed motor racing speed controls didn't even have a reverse option. Thankfully most of the brushless speed controls have the options to enable or disable reverse. You will need to check your speed control manual for instructions on how to adjust options.
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As far as how to engage reverse if your esc has this as an option and it is engaged, there are normally 2 ways to do so if you are using a "car type" transmitter with a throttle trigger and steering wheel.
Option 1. Push forward on the trigger until reverse engages.
Some speed controls just require that you push the trigger forward/away from the person controlling the vehicle and reverse will engage. Sometimes there is a short delay before reverse engages, in part to protect the transmission gears from going instantly from forward to reverse. If the vehicle is being driven forward, you may notice that when you try to engage reverse that it may apply brakes first/slow down and then go into reverse shortly after.
Option 2: Push forward for brakes/release/push forward for reverse.
This type of speed control will not go immediately into reverse if it previously was actively being driven forward (but will go into reverse if the car was sitting still). The person controlling the vehicle will need to engage the brakes/push the throttle away from the user/return the throttle to the neutral position/and then push the throttle away from the user again to engage reverse.
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If this is confusing, I would suggest downloading the manual for any of the popular ready to run vehicles as they will have detailed instructions including pictures of these concepts. The Traxxas Slash manual has decent instructions for example.
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I will add that typically you pull the throttle trigger towards your body to make a car go forward and away from you to stop/engage reverse. However it is possible to program a speed control to work the exact opposite. It is also possible to reverse the way the controls of a transmitter work buy changing either a toggle switch or programming screen on most transmitters. Without knowing all the variables, it is always best to download the manual for your particular vehicle or electronics so you can confirm how they are setup or return them to factory default settings.