第四、膝盖呈弯曲姿势。
Bend your knees and remain the posture.
youdao
第四个按扭是用来提醒你要认真写字，思想不要开小差，姿势要端正。
The fourth button is used to remind you to write seriously, thought not to wander, to correct positioning.
Fourthly, position your knees in curve shape.
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