Thank you for the clear explanation - I've seen this system referred to in the past but no-one has ever explained the mechanics to me clearly before.

How frequently during a game does this come into play ? I can see it being brilliant at key decision points, but would it get cumbersome if it was used frequently ?

In a lot of RPG's, players are rolling checks for skills, perception, etc with every action they take, and it's quicker to roll a single die, or for the player and DM/opposing player to roll one each as an opposed check. I take liberties with this, though, assigning bonuses or penalties on the fly and looking at how close the two rolls are to get a feeling for how the result should go. But your system is more considered and precise, which probably helps eliminate room for disagreement on the result.