Money Play. How should Red play 51?
Red wants White to crack his prime first. Over the board, 11/6, 8/7 was played but that was a blunder because now next turn Red will have to hit 7/1* any six. That will potentially slow White up if he stays on the bar. For a while and then it will be Red whose prime cracks first.
Best is 8/2 with 11/5 a close second. The key to the position is to keep a playable six.