Making separation happen rather than letting it happen. As 40years says it's effectively another name for good controller technique. If I sit back & watch crossing traffic miss by 7 miles then I haven't had a breakdown but if I misjudged it by 15 seconds or one aircraft changed speed or jinked left by a mile or two suddenly I have less than 5 miles. So I assign different levels or use headings to make sure I keep 5 miles.
They will definitely miss rather than I think they'll miss, without me having to watch like a hawk because I have another 20 aircraft to look after too.