When transporting equipment, VISTA Training suggests visualizing the load restraints as forming a big "X" with load, pulling in opposite directions. They should pull toward the front and the rear of the trailer at the same time as they pull from opposite sides, keeping the machine from moving in any possible direction. Loads secured to the trailer at the sides only can slide forward and backward. If secured at the front and back, it can move side to side.
Illustration courtesy of VISTA Training.