if you use the words an iron fist/hand when describing the way that somebody behaves, you mean that they treat people severely. This treatment may be hidden behind a kind appearance (the velvet glove).（温和背后的）铁拳
They promised that the army would strike with an iron fist at any resistance.他们承诺，军队将用铁拳打击任何抵抗。
The iron hand in the velvet glove approach seems to work best with this age group.天鹅绒手套中的铁腕方法似乎最适合这个年龄段的人。