vSync locks to a screen's refresh (so there are never any half-buffered images flushed to the screen).
Screens refresh at 60Hz (at least now they do, in the old days, you could get 120Hz CRTs, et cetera).
1000/60 ~= 16.6666ms / image
In the case of the GPU not being able to keep up with the screen's refresh, it will instead not send anything new, and wait until the screen's *next* refresh.
1000/30 ~= 33.3333ms / image
In this waiting, you have already dropped to the update-time of 30fps (because you're always dealing with full images, locked to full refreshes, and as such, your update speed is always going to be 1/1, 1/2, 1/3, 1/4, 1/5, et cetera).
The reason you don't actually see this in games is twofold:
1) it's not a guarantee that you're going to stay stable at a 1 image / 2 cycle update ratio (30fps).
More-likely, unless your computer sucks, it's going to be 1/1 updates interspersed with 1/2 updates.
This makes it a sporadic 30fps, rather than a stable 30fps.
2) if you're relying on fps counters, they amalgamate and average your framerates.
ie: if you have two 1/1 refreshes and one 1/2 refresh, that's a framerate of 50fps (49.98) as averaged over those three frames.
As any statistician will tell you, the earlier you start averaging your numbers, and the more often you average them together, the less accurate your finished result is going to be, and while 49.98fps technically IS the correct average of those three images (over 4 update periods), the truth is that they were refreshing either within a 60fps timeframe (16.666ms) or within a 30fps timeframe (33.333ms), and we just don't see that because of the natural smoothing caused by averages.
What's worse is that if you can't maintain a steady 60fps, this hopping causes perceived lag, while turning (first thing you'll notice).
If player updates ARE tied to vSync (horrible idea), then controls will feel jerky and unresponsive.
If player updates AREN'T tied to vSync, then controls can start feeling floaty and unresponsive (when you try to turn in another direction, really fast, but the screen still hasn't drawn a frame of your movement from three updates ago)
