High charge rates that the gas station battery chargers can do means that
hydrogen does get produced (that is the little bubbles that come up at a
rapid rate when charging with those chargers) and that means a nasty
explosion if the amount of hydrogen gets high. The thing was to allow the
hydrogen to easily get out of the area by first letting it get out of the
battery easily.
Many chargers can't make the current necessary to get the hydrogen rate up
to where it can become dangerous and the sealed batteries have chemicals to
adsorb the hydrogen and return it to the liquid by recombining it with the
other products of the charging.
It's one of those things that you get told to do for a particuar reason and
you extend it to all of that particular type of operation in a mindless
fashion.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works evevery time it is tried!