ARTCC Controller hands off two aircraft to terminal facility; with compression leading to minimum separation and corrective turn by the terminal facility.

Narrative

Aircraft X was descending to the bottom of my airspace and assigned 280 knots. Aircraft X was under NCT Approach's control. Aircraft Y was about 7 miles behind Aircraft X; descending to FL240 and reducing speed to 260 knots; but still faster by about 30 knots over the ground than Aircraft X. NCT called for control of Aircraft X when Aircraft Y was at about FL270. I shipped Aircraft Y to NCT with still about 7 miles lateral; 1500 feet vertical and 20 knots faster over the ground and slowing. It appeared to me that Aircraft X was slowed by NCT as his ground speed fell by about 40 knots; and at the same time slowed his descent and eventually leveled at FL240. Aircraft Y continued his descent while on NCT's frequency to Aircraft X's altitude and continued to have an overtake as Aircraft X had by now slowed considerably over the ground; even as Aircraft Y was also slowing. NCT eventually turned Aircraft Y with about 5 miles lateral separation. Approach control separation was never lost; and they were talking to both aircraft; but it was close for a Center Controller. My expectation was that NCT was either going to keep Aircraft X descending and going fast; or turn Aircraft Y earlier. As it was; it was a non-issue; but coordination could have been clearer between us. I could have asked NCT if they wanted anything else done before I shipped the following aircraft; or they could have let me know what they had planned.