A dense object is placed on the string before/after plucking/striking/bowing. The string on either side of the slide object vibrates. For very dense objects, long glissandi are possible.
PLACING A SLIDE OBJECT ON THE STRING DURING DECAY
The guitarists’ slide effect works by placing an object on a vibrating string to change the pitch as the sound decays. A dense slide object increases the decay time (allowing for a long glissando) and makes an overtone-richer sound. The slide only needs to rest on the string rather than press it to the fingerboard.
For an open string: the open string pitch is followed by two tones. The pitches of these are defined by the lengths of string from (nut-edge of) slide object to nut and (bridge-edge of) slide object to bridge. Either of the two tones can be dampened with the fingers.
For a stopped string: the string vibrates slide-bridge and slide-finger. The vibration between slide and finger is usually quiet and with weak overtone content.