Its principle; motility by bacterium is mostly demonstrated in a semi solid agar medium, in the medium,  bacteria swam and give a diffuse spreading growth easily recognized by the naked eye.  The medium used is sim medium(sulphide indo motility medium)  which is a combination differential medium that test for Sulphur reduction, indo production and motility. 

  The medium has a very soft consistency that allows mobile bacteria to migrate readily through them causing cloudiness.