I have a locking bar on mine that looks kind of like the pin used to secure your trailer hitch to the frame mount, but longer and skinnier and without a cotter pin. There are two pinched notches that slide into notches in the slot. Looks like this:

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Mine hold the kicker in place when making short runs so you don't have to pop up the kicker, disconnect the bar to the main motor for steering, and then redo the whole thing after you made your short run. I must say that I have only used it twice as I am concerned about additional stress that I might place in the bracket and motor.

Your motor may not be designed for this, so you may be using too much throttle in reverse. Reverse is generally only for manuevering, not full throttle travel. You could also just put your hand on top of the motor when you have this happens. best of luck. Andy