You show a rope to you audience. It looks totally normal. So you coil it around your arm to prove it.
You tend it and you recite an incantation until the rope is under your control.
Now you release one extremity, the rope is now stiff.
Of course, with a single word, the rope is flexible again.
Dimensions of the rope :
Diameter : 15 mm
Length : 90 cm (3 feet).