A man came to the town of Krasny, Russia, and publicized he would balance himself on a rope tied on both sides of a river. Rabbi Chaim Krasner, a disciple of the Baal Shem Tov, brought some of his students to watch the man perform. Rabbi Chaim's students, noticing how their teacher concentrated deeply on the man, asked why it caught his interest.
"I was contemplating how this person puts his life in danger to walk across the rope. If he would think about how much money he will receive for his act, he would surely slip and fall. The only way he can keep his balance is to free his mind from every other thought, and concentrate completely on each step. If his mind would wander for even a moment, he would fall into the river. That is the level of concentration we too must master."
(Tiferes Heyhudi, p.9; Rabbi Pliskin's Gateway to Happiness, p.151)