I remember reading about this 'trick' in the book How To Teach Your Dog To Talk. It's different than most 'tricks' in that it is a conditioned reflex, which means it is something that the animal has no control over. It is similar to Pavlov's famous experiment in that his dog was conditioned to salivate when he heard the bell, regardless of whether he was fed or even smelled food.

Here is what the book says...
You want to "catch" every sneeze, but how can you initiate one? This should be done in short sessions because you're working with a conditioned reflex.
What causes a dog to sneeze? Something that's irritating. For that reason any of the following provocations should only be used once or twice each time you practice.
1. Rubbing your hand gently over you dog's nose.
2. Tickling the nose with a feather.
3. Blowing in her nose.
4. Making her excited.
5. Continual barking, particularly when you're teaching her to start barking (speak) on command.
6. A whiff of perfume.
7. Dog grooming spray.
Use these sneeze-producing irritants judiciously. Don't use sneeze powder. It can be harmful; the above methods are kinder. The signal for "sneeze" is to move your head in a big exaggerated upward movement and rapidly lower it on the "choo" part. The verbal command is, "Ahhh-Choo!" complete with head movements.
Sounds like it would be annoying to the dog! I think "scratch" is a cuter and more humane "conditioned reflex trick".