What does 唰 (shua1) mean?
baike.com describes it as "swiftly" or "swish"... I believe it is indeed onomatopoeia.

唰 &lt;形> - Adverb 

形容迅速的样子 【swiftly】。如:唰地一下跳上墙;唰溜溜(形容极其迅速);唰啦(形容迅疾) 

Describes "swiftly" or "quickly" doing something. Like to "swiftly" jump on the wall.

唰 &lt;象> - Adjective for sound

形容迅速擦过去的声音 【swish】。如:唰唰;唰喇喇

Or describes a sound, to "swish" by something.