Once a voltage is applied, chloride (leading) ions migrate away from the glycine (trailing) ions leaving behind a zone of
lower conductivity, a higher voltage gradient, and higher pH. The zone accelerates the glycine so it keeps up with the chloride ions. As
this glycine/chloride boundary moves through the sample and stacking gel, any proteins in front are rapidly overtaken by chloride ions
which have a higher velocity. Behind the moving boundary, the proteins have a higher velocity than glycine. Therefore, the moving
boundary sweeps up the proteins, concentrating them into thin zones or stacks in order of decreasing mobility