also could anyone explain exactly what fencing does, because it seems to have changed in previous patches so not sure what info out there is correct

Yes but more importantly it works of perfect block making the perfect block window almost the same as the perfect parry window of the balanced type. This means you can make the block knife really do a lot of work since it's light attack swing speed is fast enough to get in a jab in-between majoris combo attacks and still block the follow-ups. It absolutely clowns on the tyrant when it does multiple blockable attacks. Don't sleep on block weapons just because fencing feels "safer."