My Little One (a neutered boy kitty) used to do that when he was expecting a delicious treat of canned (wet) food, but nothing ever came out.

My Biscuit (also a neutered boy kitty) does this as well (in my apartment) and nothing so far has come out. However, I know he knows how to "spray" as I have watched him doing that outside on my apartment patio where other cats come and go. I just hope he doesn't "learn" to do this in my apartment.

Biscuit's Mom