Let me try to answer for AC and he can confirm if I get it right. as you get closer to the ground the apparent speed (the one your eyes feel) will increase
in other words, far from the target, at 300 ft it does not look like 60 knots but close to the ground it feels mighty fast.
therefore if you set yourself to look rather down and keep the perceived speed constant your actual speed will precisely decay with respect to your altitude above ground.
it works well when the ground has regular features like grass, not so good good luck to make it work on snow or water.
in that case I look at the airspeed, say its 60 knots choose a feature at mid point say a strange tree, then aim for 30 knots at the tree, then repeat the process of halving the distance until the pad.