I'll take a shot
Someone more knowledgeble correct me if I am wrong.
The sail holds the boom up. Immediately below where the boom connects to the mast is the "down haul". Once the sail is up, you pull the boom down with the down haul. One end of the line is connected toward the bottom of the mast and runs through a pulley/block that is attached to the boom. Pull down on the line and that will tension the main sail.
I think that is what you are asking about. If you are asking about the boom vang, then someone else needs to help you. My E-27 with wheel steering does not have a boom vang, instead it has a double preventer. See my earlier post, where someone enlightened me as to its purpose.
Todd
1976, E-27
Alamitos Bay