What is SSO authentication touch point used for?

Question

I find there are following code in NTLMAuthenticationFilter.java which used for alfresco share SSO authenticate:

   Response remoteRes;
   if (cachedNtlm)
   {
       Connector conn = connectorService.getConnector(this.endpoint, session);
       ConnectorContext ctx = new ConnectorContext(null, getConnectionHeaders(conn));
       remoteRes = conn.call("/touch", ctx, req, null);
   }
   else
   {
       Connector conn = connectorService.getConnector(this.endpoint, AuthenticationUtil.getUserId(req),
               session);
       ConnectorContext ctx = new ConnectorContext();
       remoteRes = conn.call("/touch", ctx);
   }

and I found the alfresco/wcs/touch's description is: SSO Authentication touch point, so what is touch point used for? And how it works with SSO?

Can someone point me the direction or the references? Thanks!