are we talking with the plastic shield as chris says or the namex hook and eye way ?

as far as i know with both its the sudden stopping/slowing of the lead as it hits the water then the bait keeping going till it hits the water a fraction on a second later that gives the slack to loose the hook from the lead
the plastic shield is supposed to push the hook off to the side at the same time but i have never worked out how that works to be honest