If you do something on the fly, you do it quickly, often while you are doing something else, without preparing and without thinking too much about how it should be done.

This new rule seems to have been created on the fly. 这一新规则似乎未经考虑就匆匆制定了。

without interrupting a computer program that is already running

He had written a program that could translate from Japanese on the fly. 他写了一个程序，可即时翻译日语。