How does Jo Prove she is working on her temper when she played croquet with Laurie's British friends?

Jo proved she was mastering her temper when she kept still, quiet, and walked away to regain her composure.

From the text:

"We don't cheat in America, but you can, if you choose," said Jo angrily.

"Yankees are a deal the most tricky, everybody knows. There you go!" returned Fred, croqueting her ball far away.

Jo opened her lips to say something rude, but checked herself in time, colored up to her forehead and stood a minute, hammering down a wicket with all her might, while Fred hit the stake and declared himself out with much exultation. She went off to get her ball, and was a long time finding it among the bushes, but she came back, looking cool and quiet, and waited her turn patiently. It took several strokes to regain the place she had lost, and when she got there, the other side had nearly won, for Kate's ball was the last but one and lay near the stake.