...that your pedals should be parallel to the ground when you're not actively pedaling?

Keep your pedals parallel to the ground when not pedaling your mountain bike.

This is the most balanced and stable coasting and descending position (video addresses this at :59), and it ensures you won't knock a pedal on rocks, roots, and other obstacles. Think of your front chainring as a clock face and strive to keep your feet around 3 o'clock and 9 o'clock. 

It's also a good idea to learn to ride comfortably with each foot forward (the front foot is also sometimes called your chocolate foot). Most people naturally have a dominant foot, and will favor it in the forward position when riding downhill on a bike. But on long descents, the leg that's in front can get fatigued.

Practice on chill sections of a trail you're familiar with, switching between right and left foot forward while coasting and descending. It will feel funny at first, but over time, you'll get more comfortable with having the non-dominant foot forward — which helps balance out muscle fatigue and can be advantageous in certain scenarios. Look for more about how foot position, including how in some turns dropping the outside foot may be appropriate, in future installments of this column!