I have an alcohol burner very similar to this.
If you left the top completely open while burning, then a good deal of the alcohol will simply burn out of the top opening, and it won't be very efficient.
If you leave the top completely open for 30 seconds or so (for blooming) after lighting, and then if you cover the top completely with a metal cook pot, the burning alcohol forces its vapors out of the side jets, and that is what makes the efficient burn. This is the normal operation for a so-called low pressure side jet burner.
Now, if you had something unexpected occur, like from way too much alcohol or side jets that are too small or too few, the pressure will build up too much. It still burns great, but it might be too hot and force the side jet flames out too violently.
If that over-pressurization happened, then you would want to vent off a little of that pressure to bring it back to normal. You could do that by making the cook pot top seal leaky, or by drilling more side jets, or by making the side jets larger. If you purchased the burner, then the side jets are likely to be correct already. You could also do it by cooling off the base of the burner, but that isn't very convenient. You could also reduce the pressure by introducing a small amount of water to the alcohol, but if you get too much water in there, it will be inefficient.
I've never had one lick of trouble like this with any of my alcohol burners.
Due to the way that a low pressure side jet burner forces the flames outward, and if it shoots out too wide for a narrow cook pot, some people prefer an open top alcohol burner since the flames go upward more. It is easier to build, cheaper to buy, lighter to carry, but less efficient on fuel.