You are correct. Most propellor-driven aircraft other than very small ones
have variable pitch propellors which can change the angle of their blades
to suit particular flight conditions. For take-off, a 'fine' pitch is used
for maximum power, while a 'coarse' setting is used for cruise. If an
engine fails in flight, then the propellor is 'feathered', meaning that the
blades are aligned with the airflow to minimise the drag which would result
if the propellor were allowed to spin in the airflow.

Turboprop aircraft such as your Jetstream also have setting called 'beta'
where the angle of the blades is reversed from the normal one. This is used
in the same way as reverse thrust on jet aircraft to reduce the runway
length needed for landing, and also to reduce wear and tear on brakes. Beta
can also be used to assist in ground handling, which may involve allowing
the aircraft to reverse.

I'm surprised that they actually do this though. For one thing, aircraft
don't have rear-view or wing mirrors, and in any case most airports have
those little tug vehicles to get aircraft out of parking spots. Also, if
you reverse an aircraft and then apply the brakes, there is a risk of
tipping it up on its tail, which would be rather embarrassing.

As you might imagine, interesting things are likely to happen if you select
beta when in flight. I believe this has been the cause of a few accidents.
I don't know about the Jetstream having jets as well as the turboprops.
Maybe what you saw was an Auxiliary Power Unit (APU) - basically a
miniature jet engine - which is used to provide power to the aircraft
before engine start, but I've no idea if Jetstreams have these.