In reply to Double Knee Bar:
As Jez says, although I am not aware of a particularly clean approach short of scrambling down Cneifion Arete!
3 Options, turn left after the gate through Cwm Idwal - as you near Sub Cneifion Rib either make your way up on faint path/heather to the left of Sub Cneifion Rib or stay low and scramble over rocks and then regain path (both re-merge as you reach Cwm Cneifion, but it is rough ground. Alternatively stay on path towards slabs and scramble up gully which goes past the descent from the slabs (wet but entertaining).
It must also be possible to take a path from above the Gribin Facet which goes around the lower reaches of the Gribin Ridge, but never done it (or scramble up the Gribin Facet to reach this path)