In correlation to core design, the higher the int. diff., the more violent the reaction off the spot, to put it into layman's terms. For example, The Storm Sync and VGN are a perfect example of difference in inter diff. The Sync boasts the G2 core with numbers of 2.47 rg, .058 diff, and .028 inter diff. The VGN has the M.A.D. core which has numbers of 2.48 rg, .052 diff, and .020.
Both balls will cover about the same amount of boards in certain areas using the same covers but will yield a different reaction shape. The Sync has the potential to "corner" more violently and recover better from deeper than the VGN. As i have seen and explained, guys who cover more boards will want a higher inter diff because as they migrate inward the ball "corners" better than a lower inter diff ball. Thats the relation to an undrilled ball. But as you drill the ball, these numbers will change in correlation to how strong or weak you drill the ball.
Now, be aware that more plays into this notion, like cover, lane condition, the bowler, etc. Just because a ball yields a high inter diff doesnt mean it'll hook off the lane.