A slide rather than a strike: a fingertip — commonly the ring or middle finger — presses onto the dayan's syahi and drags across it, drawing a short rising smear of tone from the head. Friction, pressure, and path determine the contour; done well it sounds startlingly voice-like.
Ghissa belongs to the ornamental end of the vocabulary, heard in Benares solo playing and in gats where a smeared syllable marks a phrase's turn. It is rough on both fingertip and syahi, so it is deployed as seasoning, never texture.