I am reading Simon Schama's A History of Britain Volume I, which often makes reference to this form of punishment. For example, on p144:
"The assassin's made for Yorkshire, where they lived untouched for a year. Eventually excommunicated, they were sentenced to take the Cross, and some of them died en route to the Holy Land."
Or same page:
"In 1172 the pope ordered [Henry II] to take the cross for three years as penance. He never went."
What exactly did this form of punishment entail?