A muffled glissando requires muffling the strings with one hand and above that hand sliding along the same strings with the other hand. The result is a suppressed, percussive sound that, if performed fast, remindes of a muffled cluster. The dynamics goes from pp to mf.
Luciano Berio, Sequenza II (1963) © Universal Edition, London.
Yoshihisa Taïra, Sublimation (1971) © Durand, Paris.