Magriel's NYT Columns 
Consider the diagrammed position. Both sides have constructed full 6point primes. As the position stands, neither Black nor White can escape from behind the other’s prime.
The game, however, is not a stalemate. Both sides must continue to move forward until, inevitably, one or both primes break. In order to delay breaking first, each side wishes to advance as slowly as possible.
 Black to play 21. 
The correct play is 8/7, 8/6, “clearing” the 8point. Black voluntarily relinquishes his full prime before he is forced to do so! Black leads and White, with a spare man on the 12point, is quite unlikely to be forced to break his own prime on the next roll. Thus Black must plan on maintaining an effective blockage for at least one more roll. The key to an effective blockade against White’s men on the 1point is to hold the 3 to 7points as long as possible.
With the correct play Black sacrifices the 8point in order to slow down his forward progress. Black deprives himself of 6’s. That is, on his next roll, Black will not be able, legally, to play a 6, because he no longer has any men on the 8point. Sixes are ordinarily the largest, and so the worst, number for Black in terms of unwanted forward progress. Now 6’s become his best numbers — he can’t play them at all.
Many players are familiar with the idea of saving 6’s in certain holding positions. The related concept of depriving oneself of 6’s is often overlooked, even though it occurs at least as frequently. In fact, whenever one is trapped behind a prime, the opportunity to avoid playing certain numbers may be a vital consideration.
Rollout
Tom Keith 2013 

Money play White owns 2cube Black rolls 21 1296 games with VR Checker play: 2ply Cube play: 3ply Red 
21:  Game  G  BG  Equity  
1  8/7, 8/6 
W L 
.4972 .5028 
.1192 .1356 
.0059 .0079  −0.1680  
2  8/5 
W L 
.4947 .5053 
.1185 .1422 
.0060 .0090  −0.1981  (0.0301)  
3  8/7, 6/4 
W L 
.4850 .5150 
.1126 .1445 
.0059 .0092  −0.2260  (0.0580)  
4  6/3 
W L 
.4838 .5162 
.1139 .1414 
.0059 .0091  −0.2264  (0.0584) 

