Auto or manual

[squerble]

It'll probably be the the lock-up clutch on the torque converter engaging/disengaging depending on the road conditions and how the box is programmed(usual above a certain roadspeed, RPM etc.). In general you should get slightly better engine braking with the lock-up clutch engaged as there is a physical link between the engine and gearbox. With it disengaged it reverts to normal slushbox mode where there is slippage between engine and gearbox. Think driving a manual with the clutch pedal pressed part way in and causing slip, vs the clutch pedal up and no slip. On older autos you could sometimes feel the 'clunk' as it locked in, but I expect more modern stuff like yours it would be harder to notice. Maintaining higher RPM by downshifting will allow maximum engine braking by keeping the lock-up engaged for longer.

On my Focus (6 speed slush box) you can feel the lock up engage as you're accelerating in second but it's a gradual thing and actually feels like you've put your foot down a little bit more even though you haven't, but obviously this is just because more drive is being transferred to the gearbox directly as it kicks in. In normal driving when you come off the throttle the lock up seems to go to a sort of half-lockup; it's not enough to just let the revs simmer all the way down like it would, but enough that you don't feel it disengage. There's not much engine braking in general. Though it will shift down gears and lock up fully if it can't maintain cruise/limiter speed going downhill, which of course helps with the braking.