What is a St. Joseph cord?

[MWPapabear]

I purchased a cord of St. Joseph for my son. This appears to be a short, pocket version with seven knots tied to a St. Joseph the Worker medal and not the long white cord you tie around the waist. Could you provide some information about this devotion?

 

[Michelle_Arnold]

This web site page should be helpful, although I urge caution in viewing the rest of the site. Catholic Culture warns that Fish Eaters is not a reliable Catholic site (review).