Its also called a line charger. Its only good if you have a buddy connect it to your battery less than a minute before the start of your race. People used it back in the days when batteries were 1200 to 2000mah. Nowadays with batteries running at 3800mah, its practically unnecessary so long as you are running 5-6 min. stk or 19T.
If you say you're running mod it still doesn't matter because mod nowadays is a whole 'nother ball game.