It means exactly what it means "concave" it curves inward. Termed by the pro's not me!
The 9 is almost touching the denticles with the concave strike and the 7 is way closer to edge as well.
The beads on the opposite side are no longer really beads as it makes them look like denticles as well.
Somewhat similar to the 1969 nickle with machine damage doubling where the doubling of the 9's almost touch the denticle as well.