There are many reasons to lose the board control: lose of the remote, interferences, technical issues... As a consequence it can be useful to know how to brake without electronical assistance.
The most optimized method is the same as skateboarding, the footbrake. The footbrake is the simple act of braking by using your rear foot. To make this work, all you need to do is placing your foot parallel to the board and make contact with the concrete floor, starting with the heel then gradually all the sole.
Be careful to maintain your leg up to the other one, at the risk of hurt your rear wheel and lose your balance.