Just a glowing border. Put it over your current chain lightning icon and resize it. It'll light up when you should use chain lighting to gap fill. It has a custom options slider you can set to match what the sim tells you is the optimal wait time for FS/LvB.
It glows yellow when you should cast CL to gap fill for lava burst.
It glows red when you should use CL to gap fill for flame shock.
Example: your lava burst has 0.9 seconds of remaining cooldown. Your lightning bolt's cast time is 1.3 seconds. Your chain lightning's cast time is 1.0 seconds. You should use your chain lighting over your lighting bolt, because it would allow you to lava burst sooner. That's when this weakaura would glow yellow.
Example2: your flame shock has 0.9 seconds of remaining duration on your target. Your lightning bolt's cast time is 1.3 seconds. Your chain lightning's cast time is 1.0 seconds. You should use your chain lighting over your lighting bolt, because it would allow you to refresh flame shock sooner. That's when this weakaura would glow red.