With the wind coming from behind, the sail (and boat) are simply pushed forward through the water.
WIND U U +
U + U
If you hold your hand out the window of a moving car with your palm facing the wind, you can feel the wind “push” your hand back. This is how a sail works when the wind is coming from behind.
NO-SAIL (NO-GO) A sailboat cannot sail directly into the wind. You can try it, but your sails will only be luffing (flapping) and you’ll be stopped in the water... or even start moving backward. Because there is no difference in wind pressure between one side of the sail and the other, the sail cannot generate “push” or “pull.” No push...nopull...NO-SAIL (NO-GO)!