Hi Shawn
As I understand it, its that when you squeegie glass and as the squeegie goes in that s shape or whatever, depending on your technique, the water builds up on the the extremities of the blade.
As the squeegie blade moves on the glass, the corners of the rubber bend backwards and some small amount of water goes back onto the glass leaving a line tracing your squeegieing.
By bending the channel, acording to the picture, the rubber blade maintains it's shape and no water gets left on the glass.
I am no good at describing things but try bending the metal on the channel as per the picture and then when squeegieing at any speed, you will not leave lines on the glass.