'Section 79: Public Vakeel or Liar?'

I'm so confused with Section 79 of the Code of Civil Procedure, guys. So basically, if a person is aware of some material facts that would affect the disposal of the suit, they gotta tell the court or else they're liable for contempt. But then it says if they were unable to do so, they're not responsible. Unable to do so? Like, what if they just didn't wanna tell? Is there some kinda test for 'unable to' like in Article 21A for school admissions? And what's the deal with 'material facts'? Like, is it just the important ones or any facts that can somehow change court's mind? Last time I watched a cricket match, I didn't tell the umpire some material facts (like how it was an inside edge) and I got out anyway. Same thing here?