There's a guy from New York that has been promoting this concept for a number of years. 

The concept is that if you leave the stem intact, the moisture will wick out of the stem. If you crush the stem, you lose that ability to wick the moisture out.  Think about drinking soda with a straw.  As you suck on the straw, you remove the soda at the top of the straw.  More soda moves up and straw, and soda in the cup moves into the straw.  The same principle works in the alfalfa.  Moisture is drawn from the end of the stem, and moisture moves from the plant through the stem to the end, to be wick away.  That's the theory/ principle anyway.