As the expander is pulled through the clay it leaves a peel effect behind it. Turned up little bits of clay along the mole wall which are pointing downwards if you pull mole plough up hill. These then get washed away. Where as if you pull it downwards the peel effect can allow silt to build up quicker and collapse the mole. Dig a spade into clay and you’ll see it on the clay face. Usually see the peel effect when a digger bucket goes into clay.How come?