The led's draw a lot less power then the stock bulbs. That causes it to "hyper flash" because the system sees it as a bulb that's about to go out. The resistor gets spliced inline with the new switchback bulb and tricks the system into thinking that the led is a normal bulb and fixes the hyper flash. Cheap and easy install.
I ordered mine from allied electronics. I'd send them if I had any left, just gave the last 2 away. I think I have some 3 ohm if you wanted to wire them down to 1.5 ohm.