ok, I am going to do my best to explain this.

The brewers yeast trick is done by getting a gallon jug, make a water sugar solution using as much sugar as possiable that you can disolve in water and have it remain clear to slightly cloudy. leaving the jug 3/4 full, you then add brewers yeast to this mixture and place it in your grow room and leave it, within 24 hours you will see tiny bubbles forming and rising to the top, these bubbles are pure CO2 and is a byproduct of the yeast converting the sugar to alcohol. ( this is how rum is made) depending on sugar content this will continue for up to 30 days. Oh yeh do not put a lid on the jug or it will explode due to presure buildup.

If you place 1 to 4 of these in your grow room and it is fairly sealed it will feed CO2 to your plants 24/7 for up to 30 days or when you no longer see any bubbles, the peoblem is if your ventilation system has a high rate of CFM to pull out the heat in your grow room you are pulling the CO2 out before your plants can absorb it. This really works great in flowering when you go to dark and you can shutdown the exauhst fans due to you do not need them for climate control. You will need to keep your circulation fan running due to CO2 is a heavy gas and will accumulate on the floor of your grow room if no fan is running.

This is a very cheap and effective way to suppliment the CO2 to your plants.