Sentence:  "The Silver Shoes," said the Good Witch, "have wonderful powers. And one of the most curious things about them is that they can carry you to any place in the world in three steps, and each step will be made in the wink of an eye. All you have to do is to <WSD>knock</WSD> the heels together three times and command the shoes to carry you wherever you wish to go." 
Meaning of the word marked with <WSD> tags:  To bump or impact.