Its what IIRC all the 50's, 60's, and 70's models and before said on the headstocks. Its simply referring to the standard strat vintage tremolo, thats been an all the old strats and new MIM ones.
Yngwie's guitar was designed after a 1968 or 69 Fender Strat, (when CBS bought the Fender Company from Leo Fender, and they switched to the larger "CBS" headstocks that they used thoughout the 70's until 1981 I think.
I guess it means "syncronized" because all the saddles are attatched to the tremolo, so, they all move in unison.
As opposed to say, a bigsby or lyre, or anything like that that was drawn over a Tune-O-Matic (LP, SG, Gibson style bridge) because the saddles didnt move, the strings were yanked behind the saddle, and I suppose NOT syncronized.
It was just a catchword that looked good on the headstock.
ya i did see the Yngwie Strat, thats one sick guitar, it never occured to me that it was just the name for the fender style trem, i always thought that there was some difference in the fender ones that the synco was somthing special or different somehow.
There's absolutely nothing synchronised about trad Fender Trems at all - about the closest anyone ever came to a sychronised trem was Steinberger's Transtrem, which changes the pitch of all strings in the same manner - very clever!