Parents are expected to support their children to the best of their ability. This means earning to their full potential. There are really two situations where imputation can come into play.
Your ex lies about their income.
They could be self-employed and fudge their financials, they could make cash under the table, they could make significant tips or benefits, things like that. Often this comes with misrepresenting their income to CRA as well, though there are some deducations CRA allows that aren't necessarily appropriate. Here, you need to get full financial disclosure and do some forensic accounting to determine a more realistic income, or do some research to find out what a typical person in that profession makes. The imputed money would come from their actual, hidden, income.
Your ex chooses to be unemployed or underemployed.
They could be working part-time hours when full time is available, they could be attending school when they already have a perfectly good education, they could do seasonal work instead of finding normal employment, they could be doing the homemaker thing because a new spouse has a high income, they could be staying home with a child when daycare is more appropriate, they could be doing menial work instead of what their education and experience could earn them, anything of that sort. In this case it's not as clear where the imputed money would come from, though most likely they receive support from somewhere (parents, new partner, inheritance, equalization) unless they are obviously living in complete poverty.
In cases of offset child support imputation is sometimes easier. For greater earner, they simply reduce their offset payment to the imputed earner, so the money doesn't have to actually come from somewhere. For a lesser earner, the imputed person only has to come up with the offset amount, not the full amount.
Be wary though; if you are not earning to your own full potential, you may also be imputed an income.
I would also suggest that it would be wise to investigate Canlii for examples rather than ask for anecdotes here. Then you could have precedents you could use in your court case.