Science! This 1000uf 50v Low ESR Capacitor, made by Panasonic, is great for reducing the noise in your 6s setups. Ideal on setups which have little on-board filtering, this Panasonic Capacitor will add a ton of filtering might to your setup. The result will be better video, less noise introduced to your Flight Controller, and a setup which overall operates better than a setup without a capacitor. 

Setup

Solder the capacitor direct to your battery lead, either at the XT60 or, preferably, solder it directly to the primary positive and negative leads where your XT60 pigtail meet your 4-in-1 ESC. Ensure positive and negative are correct. 

Link to Oscar Liang Article about capacitors.