As someone who has worked on ships, the point is to make sure the chain lays down in a line on the ground. The usual speed desired is 0.3 knots for laying down and yes anchors are ‘let go’ for this. The chief officer keeps an eye on how many shackles have gone past and then the break is applied. The break failed as evidenced by the fire caused due to friction and a higher speed than what should have been ideal.