The quick evaluation of this position is pretty straightforward; the position is close, but the crucial play is to prevent Blue from connecting her main line with the secondary line, probably by blocking the top side.  Making a blocking move is also the obvious choice because there are no connecting or extending moves immediately available.  But which one?  Several tiles make immediate blocks.  The tiles available to block give either a YYR or a YYG, either with two candicate tiles remaining.
 
 

There are two YYR tilkes remaining. This looks superficially better for Yellow, because Yellow gets to play an additional tile.  Other things being equal, this is better than the alternative at the right.	There are two YYG tiles remaining. This looks superficially worse for Yellow, becuase blue gets to play an additional tile.  Other things being equal, this is worse for Yellow than the alternative at the right.
No Tiles can be played until endgame! And even better, if any of those tiles remain when endgame starts, they will permanantly block blue's line.	One tile can unblock before endgame.

So in this case, the general concept of pair blocking pays off bigtime!  But you can't know unless you count.  Depending on the exact tiles remaining, two blocking tiles that appear to be a tossup can be hugely different in actual effect - it all  depends on what information you bring to bear on the problem.