The relationship of hams and calves mean that by putting hams on a pre-stretch, you will feel the gastroc calf muscle more. By bending over at the hip you will stretch and feel the calves more.
So if you use the V squat machine, the shoulder pads fit nice over the lower back and you are able to load up the hips with weight.
BIG stretch, pause at the bottom, and good squeeze at the top. You can also push the knees straight so you will feel the stretch even more.
Give it a go next time you hit calves.