Look at it this way. The 4-6-3 double play has evolved into something like a ballet. The throw is coming from the direction of first base to second, where the shortstop sweeps past the bag, catching the throw and transferring, and returning the throw to first in one motion, the runner hitting the dirt during the performance. The play is moving fast. Both the fielder and the runner have the same objective: second base. In order to get the throw to first as quickly as possible, the shortstop takes liberties with tagging the second base bag. The runner will usually lose the race to the second base, and is looking for his own advantage. Here is my test..imagine there is no double play situation, no shortstop, no other player on the field except the base runner sliding into second. Does the slide look legit? In the case of Coughlan sliding into Kang, I would say yes, it was a good slide.