The Physiograph block was born out of veterinarians placing the Physiopad on top of a radiograph block to see how the horse was distributing his weight over each hoof.
On a solid block, one cannot tell where or how much weight the horse is placing on each hoof. The Physiograph Block will yield to pressure. On the Phyiopad side, it will give depending upon where the horse is placing the weight (toe, heel, evenly across the hoof), which will be evident on the radiograph.
Placing the Comfort Block side up will create a stiffer surface for the hoof but still yield to the weight distribution on the Physiopad side, which is on the ground.