Ah- Choo!se you, Pikachu.
It's kind of obnoxious when people hold back their sneezes. Not to mention, I've tried it, and who would ever want to not release that pressure?

Plus (serious concern), do you have to say "Bless You" if someone half-sneezes like that?

In my research (just now), wikipedia references the bubonic plague reason for saying bless you, but also claims that it's founded earlier than that. Other possibilities are as follows:

1: A legend holds that it was believed that the heart stops when you sneeze, and the phrase "bless you" is meant to ensure the return of life or to encourage your heart to continue beating.

2: Another version says that people used to believe that your soul can be thrown from your body when you sneeze, that sneezing otherwise opened your body to invasion by the Devil or evil spirits, or that sneezing was your body's effort to force out an invading evil spirit. Thus, "bless you" or "God bless you" is used as a sort of shield against evil.

Thus, I no longer feel guilty for not saying bless you if someone kind-of-sneezes, as it, in fact, does nothing. If they hold it back, their heart is not stopping, and the devil has no chance to enter the body.

I'm glad I figured this out. I was really worried.