I know I'd never want somebody to break their knee cap. Awful. And as far as I know, it's not an usual expression.
And I guess you should change the title. "Expression's help" sounds like that Stephen Hawking talking machine, and it seems they call those weird prhases "idioms". I'd try "help with idioms meanings".
I remember once the teacher asked us about our weekend, and a girl said "I spent saturday fooling around with my friends" and our teacher went like "I wish I had friends like you people".
:hysteria:
It seems she meant they HANGED AROUND (means ficar de bobeira, and it seems that "to hang about" means the same thing), but in the end she got us all thinking about group sex. And she was hot, to make things worse. English classes are very amusing.