Look through the ISBoxer Keymaps and see how they're setup by default. To break follow you need to create a new keymap and choose a key on your keyboard for the Hotkey. Then on the first step choose New Keystroke Action. Pick your Target [in this case All w/o Current]. Then the Key Combination is going to be the key that is pressed on the above target's window(s). This can be any movement key which breaks follow.