Single argument for mv & cp

[Clint Pachl]

The other day on the Internet I found a shell tip that showed how to use 
cp or mv with only a single argument. I tried it in the default pdksh in 
OBSD and it worked. I thought to myself, I can't believe I have been 
using the shell for over 8 years and didn't know that. Now I can't 
remember how to do it or where I found the tip. Does anybody know what 
I'm talking about? (I'm too lazy to look through source for this)

This obviously isn't correct, but it went something like this:
$ ls
test
$ cp {test,.bak}
$ ls
test    test.bak