Drain field flutes

Jon Casper is a residential builder and he wrote this article in Friday's paper.

"As a residential builder, I feel the time is now to send out a caution to all homeowners to consider covering their septic systems, preferably with hay.

With the forecast n in the upcoming week and if temperatures dip as predicted, without ground covering of about 12 inches of snow your system is at risk to freeze.  You want to make sure to cover the tank and drain field flutes with about 8 inches of loose hay.  The money you spend now is nothing compared to a freeze-up."
Of course I took it to heart.  Following directions is a skill I have.  Oh, I guess I used straw.  Oh and yes, I layer it 12-18 inches.  So much for direction following.  But as you see in my picture my drain field flutes are covered.  I wonder where and what is a drain field flute????