A rear-wheel skid in traffic is one of the more interesting experiences a rider can have. When the rear wheel stops rolling and starts sliding, the rider is still mostly in control of the bike. So long as the front wheel is turning, the bike can be steered and otherwise directionally influenced.
Note that I said "mostly" in control of the bike. Once the back tire locks up, one is pretty much required to come to a complete stop before releasing that back brake.
Here's why: when the rear wheel locks, it tends to drift to one side or another. Since the front wheel is rolling, the rider can still point the bike in a particular direction and reasonably expect to go that way. A motorcycle is an in-line sort of vehicle, however. Both wheels, with a few exceptions (yay for flat-track racing!), travel in roughly the same line when rolling.
Now, if the front wheel is rolling in one direction and the rear wheel is locked, all is well so long as the rider keeps his/her head. If, however, the rear wheel starts rolling again, the two wheels are going to align themselves correctly. Depending on speed and how much the rear wheel drifted, this realignment can be violent and lead to pain, embarassment and ugly new scratches on the bike. By "pain," I mean "broken collarbones from the high-side crash you just experienced", and by "ugly new scratches" I mean "wad of metal that used to be your pretty motorcycle." Just so we're clear.
At lower speeds or with less driftage, the realignment may just cause a moment of consternation followed by a joyous call of "Wahoo!" and a return to playing in traffic.
Yesterday on my way to work, as I tried to do something I saw in a movie once*, powerful braking was required when I tried to merge into traffic. My line of travel as I braked took me through the slip-strip (that greasy patch of car-leavin's in the middle of every lane), and my back wheel locked up as it went through the strip.
I let up on the back brake (stupid habits from stupid cars with their stupid sliding in stupid snow) and the bike did indeed realign itself. It took me a second to realize what had happened, and I did indeed yell "Wahoo!" as I continued merrily on down the road. I'm guessing the reason I got away with it was my low speed when I let off the back brake.
Anyway, I survived another ride to work, and resolved to pay better attention to my rear-brake habits. I think this weekend I'm going to find a parking lot and practice up on my emergency braking again.
*Not really. I haven't tried to do anything I saw in a movie once since that time I climbed a... uh, nevermind.