I only planted potatos once, several years ago, but didn't use this technique. I just planted them in the ground and kept mounding soil up on them as the plant grew. But this would probably be about the time that the plant part above the ground would start to dry out anyways. Did it get the flowers on the top part of the plant? If I remember correctly, once it flowers, then you'll want to cut back on watering and as the plant starts to dry up, you'll stop watering all together. This will allow for the skins on the potato to set and dry up for digging and storage.
I hope someone will come along and correct me if I'm mistaken about this.
I hope someone will come along and correct me if I'm mistaken about this.