Someone points out that David Blaine's shoelaces are untied. Blaine shakes his foot a few times, causing the laces to tie.
Alternately, Blaine may cause a seated stranger's shoelaces to tie to each other so that when the stranger stands, he trips to comedic effect.
This particular performance is not so much magic as it is puppetry. Blaine's shoelaces have a variety of wires running from themselves to Blaine's fingers. While he shakes his foot, Blaine mimes tying his shoelaces, and the wires communicate the motions to the actual laces.