I do these exercises everyday and I find the most effective way is not how many times you do it but how long each set lasts. 1 set should last 2-3mins because each rep should last 3secs when you breathe out and tighten your belly, hold your breath for 2secs and squeeze the belly, 3 secs breathe out when you relax, 1sec rest before the next rep. So each rep is 9secs and if you do 20reps then that's 3mins a set. Then you do this at least 3 times. Combine this with some kicking the boxing bag, you'll have very tight belly.

Hey Marcus! Thanks so much! You really gave us a GREAT plan!:-) It sounds great! I should try from today. So in Chinese it should be like this:
“一组动作应该持续2-3分钟，分解为：呼气并收紧腹部3秒，屏气保持腹部紧绷2秒，呼气3秒，休息1秒四个步骤，依次循环反复，共持续9秒。如果你一次做20个，那么一组就是3分钟。
每次练习至少三组才会有效，如果搭配一些拳击沙袋等练习，你的腹部会变得非常结实。”
“关键在于每组动作持续的时间长度，而非次数。”
所以总的来说，要点在于呼吸breathe.注意运动时的呼吸控制，运动效果可以事半功倍。

那我感谢你帮我学怎么用中文说。这词汇是我进常用的