Mustard gas hydrolysis (at a very high rate) Explain mustard gas hydrolysis at a very high rate:
  
  $\ce{Cl-CH2-CH2-S-CH2-CH2-Cl}$


I tried searching online but did not find this reaction anywhere. Can someone explain me the steps/mechanism of this reaction? Even some hints might help.

My attempt:

According to Wikipedia 


  Solvolysis is a special type of nucleophilic substitution ($\mathrm{S_N1}$) or
  elimination where the nucleophile is a solvent molecule. For certain
  nucleophiles, there are specific terms for the type of solvolysis
  reaction. For water, the term is hydrolysis; for alcohols, it is
  alcoholysis; for ammonia, it is ammonolysis; for glycols, it is
  glycolysis; for amines, it is aminolysis.


It clearly states that hydrolysis/solvolysis can only be $\mathrm{S_N1}$ or elimination. However I'm wondering how can $\mathrm{S_N1}$ occur on a primary carbon! Morever water doesnt seem to be a strong enough base for elimination. So what will be the preferred reaction mechanism?
Luckily I got the mechanism via Google Image Search.



Internal attack by sulphur atom takes place during the reaction. This is called anchimeric assistance, neighbouring group effect or $\mathrm{S_N{NGP}}$. Wikipedia describes it well.