Here's an article about rev matching on a manual trans 370z

2009 Nissan 370Z's SynchroRev Match Explained - Tech Dept. - Auto Reviews - Car and Driver

The idea is that in a manual trans, when you downshift, you tap the throttle while the clutch is in, just before releasing the clutch after downshifting. This is so the car doesn't lurch because otherwise, the momentum of the vehicle, transferred to the engine through the wheels, clutch, driveshaft, etc., is what causes the rpm to jump when the clutch is engaged. Instead, the engine already is at the proper rpm because you've tapped the throttle. When one matches the rev, the shift is smooth and the clutch & transmission is spared unnecessary wear. According to the article, Nissan made things easy -- in the stick, the engine *automatically* rev matches so you don't have to tap the throttle.

But Nissan didn't stop there. They have rev matching in an *automatic* when you manually shift. This guy shows how it works is his 370z:

YouTube - 370Z with automatic transmission Rev match explained

This brings me to the point about jerky downshifts in DS mode when you hit the brake (more than lightly). I suspect, but can't say I know, that rev matching does not happen simply by braking in DS -- because you are not manually downshifting. Instead, the car seems to downshift, but only to a gear which will boost the rpms, since this is sport mode. And it doesn't seem to rev match, since you are on the brakes. At least this would help explain why the downshift is not smooth. Thoughts?

I note that when you brake in D mode, the clutch seems to remain disengaged (just like on any old automatic) until you lift on the brake, at which point it finds the right gear at a low rpm -- after all, you're in D mode and having a senior moment, so there' no need to be all strung out at 4 grand, ready for battle.

 

Speaking from an AT perspective, I think the jerkiness felt is the engine braking associated with the lower gear. I have experienced the DS type downshifting in D when I have a "panic" type stop.