[Christ]

Precisely.

Also, the vacuum is trying to pull air down past the compression rings, so they seal better against the cylinder walls, meaning less opportunity for "blow-by" in either direction. Less combustion gasses leaking into the oil, less oil in the combustion chamber, less compresssion loss before combustion, etc.

So basically, it's a combination of small boosts that end up being a decent boost in HP, and, at least in theory, economy.