I do not know the exact difference in the force required to break boards with or without spacers, but it is many time easier to break with spacers, that is why people use them. You know how easy it is to break one board; children do it, old Senators do it, and television talk show hosts do it. With spacers, you are breaking one board at a time; each board is no harder to break than the one before it. If you break the first board, you will break every subsequent board as long as you sustain that same amount of force. Therefore, if you could maintain the amount of force required to break one board over the distance of a yard, you could break about 36 boards with spacers. Humans are not able to sustain that amount of force over that great a distance; so they hit the first board with as much force as possible and try to maintain the force until it dwindles to the point it will no longer break a board. To break boards with spacers, your hand or foot must move though all the boards with a sustained, relatively low level of force. In the world of martial art training, self-defense, professional fighting, or street fighting this type of force is useless.