I prayed using a [Siddur Tehillat Hashem][1] the other day, and I noticed that many more [shvas][2] were marked with an asterisk above, indicating that they should be vocalized, than I'd expect. In particular, there were many cases in which a shva following an unaccented "short vowel" and not under a letter with a [dagesh][3] got an asterisk.

One example, which I confirmed is not marked for vocalization in the [Artscroll Siddur][4] (for example), is the last word of the second verse of [Psalms 104][5] (AKA "Barechi Nafshi"): כַּיְרִיעָֽה, which would be pronounced like "kayeriy'a" according to Tehillat Hashem and like "kayriy'a" according to Artscroll.

Where does this distinction come from?

  [1]: http://www.judaism.com/seriesdisplay.asp?USN=231
  [2]: http://en.wikipedia.org/wiki/Shva
  [3]: http://en.wikipedia.org/wiki/Dagesh
  [4]: http://www.artscroll.com/Categories/pbk.html
  [5]: http://mechon-mamre.org/c/ct/c26a4.htm