This is an annoying unit with instructions which contradict themselves. The table on Page 3 clearly states that the unit has NONE under the BEC column heading, yet the photo on Page 4 of the set-up shows no separate receiver pack - which implies that it does have BEC, or something very similar in function. I did find reference to a cut-off voltage setting, though (Page 8/9). This involves using the "optional programmer". If the voltage is set too low for your drive battery then as soon as you put it under load the battery voltage will drop below the ESC drop-out setting and guess what?
The saving grace is that this is a British unit so I would advise contacting Fusion and asking them to explain to you how to sort it out. It must be galling to find out, having spent well in excess of a hundred quid for the thing, that you may need an "optional" programmer to get it to work.
DM