An undercut is used when the load is lifted on the point of the forks rather than back at the heel (closest to the mast). This is done when the load is to be put down close to a wall or another item. If the load was heeled, the forks would be in the way and would either waste space or puncture the adjacent item. When undercutting, the maximum possible weight is reduced in the same way as you could lift a heavier item close to your body than at arms length.