The vang, though it’s action seems fairly straightforward, is yet another rarely used sail trim control on sailing vessels.
The main purpose of the vang is to keep the boom from rising on downwind points of sail and especially on the deep broad reaches. Preventing the boom from rising avoids too much twist in the sail and maintains efficiency and power.
Some tension the vang on a close-hauled point of sail but this is usually unnecessary as the mainsheet should be sufficient. I’ve even had to detension the vang when close reaching to allow that top telltale to flutter backwards! Let the main do the work.
Technically, one should have a sliding scale of vang required the farther out the mainsheet is played as such leads to increased mainsail twist. Realistically, most who use the vang probably tension the device as they are assuming a near deep broad reach. Again, the vang is needed to prevent excessive twist of the main in this point of sail as the mainsheet is unable to do the job of keeping the boom down.
The vang can be released suddenly to depower the mainsail by allowing excessive twist.
There are many different types of vangs on sailboats. I don’t understand them but they all do about the same. Learn to use the type of vang that you have effectively.