expert for the court. he s testified in many other cases. nel is shrewdly trying to get the persuasive value diluted. reporter: the key to the testimony is really trying to strike at core parts of the state case. absolutely. and you can see this by the face that nel kept him on the stand for two days. he realizes the importance of devaluating that evidence. we ve seen him put evidence at the sound of the cricket bat and could that have been the cricket bat mistaken by the gunshots by the ear witnesses which would mean that they, in fact, heard pistorius screaming and not steenkamp. and also in terms of ballistics. dixon suggesting that the bullets were fired in quick succession, supporting pistorius version of events, rather than the state saying he shot slowly and, therefore, intended to kill. core parts of the case being called into question. again, robyn curnow speaking with cnn legal analyst kelly phelps. when we come back, the world