Originally Posted by Nissan USA

SynchroRev Match can help with upshifts by holding the RPMs when the clutch is disengaged rather than dropping them to idle like a normal car would.

For drivers who take longer to shift, this can help them be smoother since the revs won't drop down to idle immediately.

Of course, if you wait too long I believe the revs will go to idle, but as soon as the gear shift lever is moved in the direction of a gear the system will set the revs at the appropriate RPM for that gear.

Sorry for the lengthy explanation...it's much easier to understand if you have a chance to experience it.