The concept of speech level singing is to produce your tones at speech level ie: with the ease of speaking.
SLS is a technique where the singer is taught to use less vocal energy in approaching the act of singing (especially belting). With practice, the singer learns to mix more head voice into the singing. This lessens the wear and tear on the voice. Think Sinatra; Think Ella Fitzgerald - their singing is easygoing, unstressed, and puts little pressure on the larynx.
Speech level singing uses special exercises that dis-engage the muscles located outside the larynx. This will allow the more efficient, inner larynx muscles to do what they do best! They will control all the vocal chord adjustments AND breathing and correct resonance will happen naturally.
Speech level singing is a technique used by many professional singers. Names include Michael Jackson, Jennifer Lopez, and many many more. A recent survey that consulted few of the professional singers in reviewing the programs are SLS graduates, and swear by it.