Sit Back Meaning

(idiomatic) To relax, to not exert oneself.

Example: To recline while still in a seated position, with one's back on the frame of the seat.
  Please just sit back and watch the show.
2010 December 28, Marc Vesty, “Stoke 0 - 2 Fulham”[1], BBC:
  Predictably in the second half it was the Potters, who went searching for goals as Fulham sat back to defend their lead.