動詞のshagはここでは採り上げにくい意味を持つ単語として辞書にはありますが，その他に「球拾いをする」（英辞郎 on the WEB）という意味があります。よってshag ballsは「球拾いをする」と解釈できます。（Koyamamoto）
The dark side of the Koshien dream
Player development suffers due to dearth of coaching education and a win-at-all-costs mentality
Imagine the Boston Red Sox abandoning Fenway Park for over two straight weeks mid-season to make way for high schoolers. That’s what happens every August here in Japan, as the Hanshin Tigers, the nation’s second most-popular professional team, pack their bags and go on the road, giving up their home-field advantage — and their field — for high school baseball. This year, the familiar opening siren will sound to mark the start of the 96th national tournament on Saturday.
… What will the other 80 players do? Shag balls. Some high school players from the powerhouse teams have not played even one game, while the top 20 play nearly 100 games during their year-long season. Late developers should get to practice and need some game exposure, since they may become good enough to start in the near future if only given the chance. One of the most admired sportsmen of all time, Michael Jordan, was a famous late bloomer.