I must mention that this is not something that is new in Quebec. Université Laval had this in 1986, so they were the precursors.
What happens is that, when a woman comes back to work after maternity leave, eventually asks for a promotion to the next rank and obtains that rank, the promotion is granted retroactively to compensate for the maternity leave. If she took a one-year maternity leave, she would be granted the promotion at the start of the maternity leave, not at the end of the maternity leave, and she would receive compensation for the year she lost in terms of the salary increase. This is because, when you move from assistant to associate professor, there's an increase in salary.
She would have the retroactive pay of that year she was on maternity leave when she finally applies for the promotion. That's what I mean by retroactive. The maternity leave is not penalized.