Boundaries are counted obsessively while the twos that separate good chasing sides from average ones go unrecorded.
The second run is the one nobody trains for
Count the twos. Not the fours, not the sixes, not the strike rate. Just the twos, across a chase, and then compare the two sides.
I have done this often enough to be confident about the pattern. The side that wins tight games usually converts more ones into twos in the middle overs, and the gap is not small. Fifteen extra twos across an innings is fifteen runs, which is most of a normal margin of victory, and it arrives without a single risk taken against the bowler.
The reason nobody talks about it is that boundary counting is easy and running is not measurable from the scorecard. A two looks like two runs. It does not look like what it actually is, which is a decision made in about a fifth of a second by the striker as the ball passes the fielder, based on where that fielder is standing, which shoulder he throws off, and whether the non striker has backed up far enough to make the turn possible.
Fitness gets blamed for the absence of twos and I think that is mostly wrong. The best running pairs are not the fittest players in the side, they are the ones who have batted together for years and turn without looking. The limiting factor is agreement, not lungs. A batter who has to check whether his partner is coming has already lost the second run, because the decision window closes while he is turning his head.
The deeper cost is what it does to the bowler. A side that runs hard forces the fielding captain to hold his sweepers deep, which shrinks the gaps in the ring, which makes the ones easier, which is the loop the good sides live in. A side that does not run lets the captain bring everyone up and squeeze, and then you get the middle over collapse where nobody has hit a boundary in five overs and somebody plays a terrible shot.
I would spend more practice time on this than on hitting into the stands, and I know exactly how that sounds. But the sixes are already there. The twos are lying on the ground unclaimed, in every single game, and taking them requires nothing except two people who trust each other enough not to hesitate.

