Green’s Dictionary of Slang

synch n.

In phrases

in synch (also in sync) [SE phr. in synchronization, of sound, usu. in films]

fig., in tune with.

1971		H.E. Roberts Third Ear n.p.: in sync adv. in line with; according to.
1983		R. Price Breaks 69: The place was totally in synch with my state.
1993		Tarantino & Avery Pulp Fiction [film script] 50: Vincent and Mia seem to be [...] shaking their asses in sync.
2015		(con. 1919) C. Fountain Betrayal 63: Johnson the steady hand on the tiller, Comiskey in sync with his friend’s vision.