It is necessary to pay attention to the feet when the worshiper repeats
"Holy! holy! holy!" and he is to lift up his eyes toward heaven. At the
instant the Kiddushah is repeated he needs only lift up his heels, and
thereby his body from the earth toward heaven.... According to Tanchuma
it is necessary to lift up the feet from the earth altogether, after the
example of the angels, of whom it is written (Isa. vi. 2), "And with two
he did fly." It is from this text that the sages have ordained that a
man should fly up (as it were) when he repeats "Holy! holy! holy!" And
let the chooser choose, i.e. it is optional either to lift up the heels
only or to jump.
THE KABBALA, _Kitzur Sh'lh_, fol. 48, col. 2, and fol. 49, col. 1.