This add-in circuit enables the taillight strip to work like an 1157 bulb. So, the light will be a running and brake light or a running and turn signal indicator.
Aka: changing the light from a 2 wire setup to a 3 wire setup for running light and brake light
Figure out how many LEDs will be used and choose the dual element circuit will work
15-25 LEDs part number 6120-00
30-60 LEDs part number 6122-00
65-125 LEDs part number 6121-00
130-150 LEDs part number 6124-00