Why Did Umpire Give Rassie Van der Dussen Not Out Despite Wickets Hitting?

During the 1st T20I match between India and South Africa at Delhi's Arun Jaitley Stadium, an incident occurred in the 19th over of the run chase where Rassie van der Dussen was given not out by the onfield umpire for an LBW appeal by the desperate Indian players. 

What happened on that delivery? 

Van der Dussen came down the track and tried to flick the ball, however, the ball missed his bat and directly struck his pads. Visually it looked plumb since Van der Dussen came across but the umpire gave it not out because of the Clause 3.4.6.4 under Appendix D of the ICC men’s T20I Playing Conditions, which says that: “When a not out decision is being reviewed, where the evidence shows that the ball was hitting, the point of the first interception was in line, and the ball pitched in line or outside off, but that the point of the first interception was 300cm or more from the stumps, the on-field decision shall stand (that is, not out).”

Rishabh Pant reviewed the decision which was fair enough. And upon the review, it was confirmed that the point of interception of the ball was more than 3 meters and the decision shall stay not out. 

What is this 3-meter rule in the LBW law? 

Clause 3.4.6.4: "Where a Not out decision is being reviewed, the ball-tracking technology must report that the ball was hitting for the batter to be eligible to be given Out, otherwise the batter shall remain Not out. However, where the evidence shows that the ball was Hitting, the point of first interception was in Line, and the ball pitched In Line or Outside Off, but that:

• The point of first interception was 300cm or more from the stumps; or
• The point of first interception was more than 250cm but less than 300cm from the stumps and the distance between the point of pitching and the point of first interception was less than 40cm, the on-field decision shall stand (that is, Not out).

This rule is there because of the fact that the DRS system which is built cannot correctly estimate the height of the ball when the point of interception is more than 300 cm or is 'more than 250cm but less than 300cm from the stumps and the distance between the point of pitching and the point of first interception was less than 40cm'. 

It is a physics and a projectile problem that the system has and the rule made to counter that problem. Warren Brennan, the founder and chief technology officer of BBG Sports where he has co-developed the Hotspot/RTS system, said, "when the ball is very deep on the crease and you might only get 30 or 40 centimeters before it hits the pad it's almost impossible to get a good trajectory," on the Cricketingview podcast. 

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All the protocols around the decision-making for such events are made taking into account such physics problems. It isn't easy to just get rid of these clauses because there is a sophisticated system built that does have its limitations but still produces great results to keep the decision-making intact.