Tapping this button tells BpmDj to shift the second song up/down and left/right to find the best beat alignment. A maximum shift of 4 bars is allowed in either direction.
Although the automixer works well on repetitive music without too much vocals, it is blissfully unaware off the meaning off a vocal track. Actually, the software doesn't even know a certain energy is vocal or not. That means, that regardless of the meaning we assign to music, he will place them over each other. In case you have two singers, they will very likely bump into each others space if you automix them.
The automixer is also unaware of the first beat of a phrase, nor does it recognize the first bar in a phrase. If you want it to take this into consideration you should find an appropriate break and include that in the transition you want automixed.