All the discussion aside. Why would you like to spend XPs on a talent, that all it does is to allow you to spend a triumph to knock people prone, when they may spend 1 manoeuvre to stand up?
It's possible to take an initiative slot before they act again and gain the benefits of them being prone.
That said, it isn't much in the way of benefit. It's one boost die, which you could give yourself with 2 advantage.
I think part the issue here is that knocking someone down doesn't do much unless you have a group of melee people engaged who can take advantage of it. Making a whole talent around it seems like a waste.
Maybe a cleaner house rule would be: "When you score a critical in melee combat, the target is also knocked down."