Rob,
By default, all RDs scan (ie sweep) the entire Ka band radar spectrum - which ranges from 26.5 to 40 GHz. As a RD is essentially a computer, the unit takes a certain amount of time to sweep the entire spectrum. It has been found that with police frequently using very quick bursts of instant-on (I/O) radar, that an RD can miss such a shot because it was sweeping a different area of the spectrum at the time the radar signal was sent. The (original) Escort Redline earned the name "sleeping beauty" because of this - it was vulnerable and completely missed some very short I/O shots.
What sweeps / band segmentation does is allow the user to select certain ranges of the spectrum that the RD will scan, so it can ignore the rest. Police Ka radar only operates on a few select frequencies - in North America those are 33.8, 34.7, and 35.5. So, the user can set his RD to sweep only narrow ranges around each of these specific frequencies, without it wasting its time scanning frequencies that are not used by police. This allows the computer in the RD to make smaller, faster, and more focused sweeps thus making the RD much more reactive to very short bursts.
Also, by setting "custom sweeps" (Valentine 1) or using "band segmentation" (Escort / Beltronics), Ka falses are also reduced. For example, cheapo detectors such as Cobras are notorious for "leaking" Ka band in the 33.6 - 33.7 range. Anyone with a V1 may have observed it give a Ka alert followed by a weird sound and a "J" on the display. This is the V1 detecting a "junk" detector and identifying it as such. Using custom sweeps also help improves the overall reactivity because the unit can be told to ignore the typical false frequencies used by Cobras allowing the built-in filtering to be turned off. With a V1 this filtering is called Ka Guard and with Escort/Beltronics it is called RDR (Radar Detector Rejection).
Not only does this improve the reactivity (speed) of the RD, it has also been proven to improve, sometimes significantly, the sensitivity (range) of the RD.
These options are only available in a few high-end units (Beltronics STiR-Plus, newer V1, Escort Redline), but it is WELL worth it. Seriously, if you are going to buy a RD, spend the money and buy a high-end model. The original Redline has been turned into the hottest RD on the market, simply by a firmware update that added band segmentation.
Here's a video of the benefits of the V1's arrows and its rear antenna horn. As you can see, both my STiR-Plus and the V1 alert at exactly the same time to a very weak I/O shot (over 4 miles ahead). I slowed down and as I eventually went beneath the underpass, I was shot in the rear by a cop hiding up behind me on the on-ramp. Note how the V1 alerted almost 6 seconds before the STiR-Plus did (and the Plus is the top of the line custom installed unit!), and the arrow told me the cop was behind me. Fortunately I had 2 high-end units that detected the very first alert (lesser models would have missed it) and I was slowed down. With lesser units, I would have likely driven right into the trap (or sped right back up after the first alert was over) = a ticket for ~30 over.
If I didn't have the V1, when the cop shot me I would have received only a relatively weak alert from the STiR-Plus (a reflected signal) and I would have been scouring the road ahead trying to find out where the cop was. In this instance, the strength of the V1's rear alert and the arrows told me to look over my shoulder, and I knew exactly where he was.
I run both the (band segmented) STiR-Plus and the (custom sweep) V1 on the highway, and I am well-protected. Maybe overkill, but after experiencing the benefits of the V1s rear antenna/arrows, I won't leave home without it and it is an excellent compliment to the STiR-Plus.
Sorry for the long post.