I think it is a reference to a military expression, to shots fired in anger (in an armed conflict) as opposed to in training. The suggestion is to test the brakes while shopping for the bike and then practice with them to become confident with them before you need to use them in a situation such as you describe. Seems to me that you got a little too close to the black hole on one of your first outings and passed through the event horizon!

How fast were you going? How steep was the descent? Were you on a road or off road? Was the surface smooth or rough? Your bike may have a problem, but maybe you just need more experience with the type of terrain you were on or the speed you were going. When you get back on the road approach similar conditions with more caution until you are confident you can handle them. And use your brakes!

Ken