On this map, you can see the Matilda Champion House at the very top of the map, and the position of Hovey's guns, which would have been near the pond closest to Champion Hill in a line running perpendicular to the peak of the hill, facing up the hill.
When Boomer's left was forced backwards for the third time, the gap that opened up between Boomer (now facing east) and Holmes (facing southwest) allowed the federal artillery to finally target the Confederate lines.