The full eight-minute National Park Service safety video for Big Bend Park in Texas:

"This Video Could Save Your Life" by Reine Wonite,

is now back online, and can be viewed and/or downloaded here.

Since the signal mirror flash is 5 minutes into the video, I've extracted it and posted it on YouTube here:
Aerial Signal Flash Footage at YouTube
where you can view it at different sizes and resolutions, forward the link, etc.

I've also embedded a lo-res version here -
it pales compared to the 1080P full-screen version you can see on YouTube, but is very convenient:



I like it because it is (by far) the crispest aerial footage of a signal mirror flash that I've seen.