This is the RotY curve for one ball (not lattice). It does two complete rotations (0-360) then rotates to about 500 degrees with a slow transitional fall-off. The small flat spots in the first two cycles are where the ball hits the floor and rotation pauses for a split second. The trick is to carefully align the rotation stop-point with the stop-point on the lattice’s Location IPO. (I think there is a script which will do some of this but I like doing stuff the hard way :))
You could achieve the same result with one curve from 0 to 1200 degrees approx but I like to see the individual rotation cycles.