It's a voiceless lateral fricative: the tongue-tip rests against the upper gum, and the air is forced down the side (or both sides) of the mouth between the tongue and the upper side teeth. The effect is not unlike "th": British English speakers often substitute "thl" for Welsh <ll> in placenames ("Thlandudno").
----- Original Message -----
From: markodegard2000
Sent: Tuesday, April 30, 2002 7:30 AM
Subject: [phoNet] Re: Dental Work.

My tongue has always tended to stroke the upper side teeth when making
these fricatives. The tongue is spread wide at the front of the
mouth, resting just under the teeth, almost as if I was
going to 'scratch' my tongue with my upper teeth. I forget what Welsh
orthographic ll stands for; for some reason, my memory insists it
should be like the double L in llama, but this cannot be correct.