What Bob said...
Until you can go nice and smooth like this; keep practicing and save the pedaling until proficient
I think keeping the upper body relaxed, non-tensed is also a key. The shoulders and arms should be relaxed...keep in mind there is a difference in firm grip and "death grip". "Death Grip" in my mind is when the whole upper body is ridge and tense causing unwanted feedback to the handlebars and down the boom triggering unwanted twitchyness and wobble. It's not recommended to unpack, build, and hit a fast downhill on your new Cruzbike. Practice the flats first, build the confidence, then tackle ascents and descent, start out small, work you way up. It's easy but does require patience and practice.
Foot and leg position, is more a matter of taste and what makes you comfortable. At this point I can rotate my pedals slowly or fast, lock my legs or keep them relaxed. I generally keep them relaxed, and may lock myself in if I see a big bump ahead. For me, I keep my whole body relaxed and just go with the flow of the bike with a simi-firm grip.
Here is some video I shot March 4 2015 on the Vendetta coming off one of the most exciting mountains in my area. Most speeds are in the 40 mph range all the way up to 48.3m mph. It was not a 100% coast-job, I did do a lot of braking as the speeds can get quite fast through the turns, and not able to see what's around the corner. The scariest but most exhilarating part for me was the last few seconds, coming into a corner and maintaining 41-42 mph through the corner drifting towards the guard rail.
Note: It should be noted in Bob's video above, he did that one handed!
Strava info