The muted strum is executed by relaxing the chord hand during the strum. If the chord is a barre, relaxing the pressure is enough to create the percussive, drum like effect. If holding an open chord, the fingers relax but maintain enough finger pressure to get the muted sound. This is what a few players are calling, The Universal Muted Chord. You're utilizing your ukulele as a percussive instrument and there is no discernible pitch - just like a drum. This chord works on any song just as a drum works with any song and does't care what the key is. This is a great chord to use when learning a new strum.