Each song is mixed tightly because the software: Mixes songs at the right points. Uses beat matching to keep beats synchronous. Uses a beat filter during mixing. Has gain control (each song plays at the same volume). Has options to find the right combination of songs that always sounds harmonious when mixed together.
And that's not all: thanks to an ingenious algorithm, the player can mix your entire music collection without changing the pitch too much. In this way songs with large differences in bpm are also included in the playlist. And it's also possible to save and broadcast mixes (Icecast and Shoutcast).