What is the mechanism behind "acquired" alcohol tolerance?
The subject of alcohol tolerance is broader than it might appear. However, in most cases people refer to what is called metabolic alcohol tolerance. Based on your response to the comment made by Alexander Galkin, I assume that you also refer to metabolic alcohol tolerance.

Metabolic alcohol tolerance is the result of drinking significant amounts of alcohol over a period of time as you describe. Both the amount of alcohol and the duration that it takes to develop an increased metabolic tolerance towards alcohol differ from person to person.

When most people drink alcohol, the liver will responds by activating alcohol degrading enzymes that break down the alcohol molecules, which results in the elimination of the intoxicating effect. The activation of those enzymes occurs much faster with people who have developed metabolic alcohol tolerance and the concentration of those enzymes is generally higher. This means that those people can break down alcohol faster than you and me, that is, they can "tolerate" more alcohol in the sense that they simply get rid of it much quicker.

On a final note, being more alcohol tolerant is not exactly correct as metabolic tolerance doesn't necessarily mean that the organs affected/intoxicated (such as the brain) by alcohol aren't necessarily more tolerant. The alcohol is simply broken down much faster and can therefore have a limited impact.