Here is an interesting link which turns the recent business of Palestinian slingers 'on its head'... apparently at least some Israeli soldiers could use the sling.
  http://www.ucalgary.ca/~elsegal/Shokel/880527_Rock4Rock.html

i was reading this one fantasy book were they used staff slings to sling crude hand grenades at people

Quote:

Since the writer records that Desta showed how to cause the sling to make a swooshing sound, I believe it is justifiable to conclude that Desta then showed how to turn off the swooshing sound.  We are therefore not talking about the whip-crack (or usually, in my case, the *whuff*) from the end of the sling cord.

That swooshing sound seems to relate to the stiffness of my cords.  My dacron sling cords are stiffer, and louder, than my nylon sling cords.  The sling cords I braided myself from twine, are quite pliable and very quiet.  I am wondering if Desta just worked the cords until they were no longer stiff.

"Turn off the sound" may be a poor choice of words, since (at least to mechanically-minded folks like me) it implies an ability to turn it ON again.  It may be that once quieted, the cords cannot be made noisy again.