Actually, it should drop straight down as it is moving slightly forward. This gives a kind of "spiraling" or "drilling" action to the Tan Sao. If you are moving to the side, then something is wrong. What Hendrik is referring to is what SPM would call a "Choc Sao." It looks like a Tan Sao, but circles outward and slightly back to "hook" the incoming strike off of the centerline. Two very different "force lines", as Hendrik says, though outwardly the hand form looks almost the same.