Some things to note, Digital Mixing is the emulation of the old style Vinyl Mixing where as a basis, two or more songs have to be carefully chosen. With the turntables, you have the pitch control which also is present on most DJ Software as in Traktor. There is also a Sync button which actually acts the same as the pitch control but it uses a master Deck with an active song playing as a reference to adjust the second track and other tracks automatically, when deployed.
Not because two tracks are the same BPM or Synced means that they will work well together, however they can still be mixed if the DJ is skillful enough to do so manually (Slip Cue). The Sync button is not just a push, play and forget button. You have to know how and when to engage it. Playing one song after the other from beginning to end maybe with a fade between is what is called Push Play. Continuous play, which is also a feature in Traktor can also be considered as "Push Play". If you engaged the sync button and do not readjust it at the end of the song, the next track on the other deck can be synced in the wrong way and thus speed up or slow down the new track in an undesirable manner.
To summarize, before Mixing, Beatmatching and Syncing, what we had was just "Push Play". A Record was just loaded on a platter, CD in a deck or (in the case of DJ software), track in a playlist. Song after song would be played without a seamless blend or transition. So the sync button is not a magic answer to mixing, just a digital aid. There are some who have DJ Software and just push play and play music without knowing the full capabilities of their software however the art of beatmixing and transitioning needs to be first learned in the mind before it can be aided by Hardware or Software. It is a creative skill. Some know how to do it, some don't and do not care about learning it and then there are those who just do not have the knack for learning or doing it.
