Soft Signal: The rule under which Suryakumar Yadav was given out

The debate has started after Indian batsman Suryakumar Yadav was given out under the “soft-signal” rule in a T20 international cricket match played against England on Thursday.
 Yadav played 57 innings in his very first match.  In the 14th over, he fell victim to Sam Karin.  Playing a pull shot, he catches David Malan at deep square leg.
 On-field umpire KN Ananthapadmanabhan asked TV umpire Virender Sharma to pass judgment and gave a ‘soft signal’ of the out.
 But upon seeing the replays it seemed that the ball had touched the grass.  The third umpire was not fully convinced and upheld the decision of the field umpire Anantpadmanabhan under the soft signal rule.
 What is a soft signal?
 According to International Cricket Council (ICC) rules, a “soft signal” is a gesture that an on-field umpire makes before making any decision to the third umpire.  In this, he gives an indication of what his decision is in that situation, whether he is considering the batsman out or not.