“...air out of your BC”?
Not being a diver, I don’t know what a BC is, and I assume you don’t mean “Birth Control”. ;-P
Please elucidate...
BC = Buoyancy Control
Its the air bladder divers wear. You can pump air in to give yourself more buoyancy or let air out to have less.
Works kinda like an elevator. Properly used you can remain at the same depth without any work. Being a novice diver she most likely did not have good control.