what is happening is that that plane has a Low voltage cutoff safety. What this does it cuts power to your motor when you have drained your battery to a certain point so that way you will know that it is time for you to land. By cutting the engine you know to land and it leaves you enough power to control the model.

By throttling all the way down and comming back up resets this feature and allows you to add power if you need it to correct on a landing.

does that make sense?