I'll save you a bit of trouble
That is a Kick module. They insert into the mod between the battery and the top electrical pin with the white dial facing AWAY from the battery. The white dial allows you to set a wattage setting between 5 and 10 watts (12 watts if it's a Smoktech clone). It will then maintain that wattage output. That's very important when using an over-the-counter topper like a Cartotank because sometimes the standard 3.7 volts just isn't enough power for the coils, and it prevents the power dropping (weaker vape) as the charge is depleted.
The Kick also acts as a safety device which is important if you get a bad coil or a short. It will protect your battery, and you.
The Kick will not fire if the coil resistance is less than 1.0 ohm.
The Kick will not fire if the amp draw exceeds 5A. (6A for the Smoktech Clone)
The Kick will not fire if the battery voltage drops below 3.2V.
Kick modules run about $15 or so.
The Kick is the same as the Kick, except it will go up to 15 watts, will go as low as 0.5ohm coils, and will allow up to 6 Amps. Kick 2 costs about $45.
If you're not subohm vaping then the standard Kick will be all you will need.