Clip 5 - 28 November 2017 - Law 22
A momentum try.
From a tackle/ruck, the bal comes back to Danny Care, England's scrumhalf. He immediately kicks low into empty space to his left where the ball bounces on. Jonathan Joseph, England's outside centre chases after the ball as Reece Hodges and Samu Kerevi race back in defence. Joseph beats them to the ball as he dives onto it about seven metres from the goal-line. He rolls an d slides as Kerevi and Hodges try to stop him. Joseph slides over the line where he grounds the ball. The referee awards the try.
Law 22.4 Other ways to score a try
(d) Momentum try. If an attacking player with the ball is tackled short of the goal line but the player’s momentum carries the player in a continuous movement along the ground into the opponents’ in-goal, and the player is first to ground the ball, a try is scored.
Joseph's body does not stop moving forward in the wet. He does not propel himself along by using arms or legs. It is a "continuous movement", and so the referee correctly awards the try.