Hackney and cob training aids - educate me please

[pennyturner]

I'm aware that there are various training methods to encourage show cobs and hackneys to lift their feet. The janglers I get - it makes a noise as they step. However saw this:

http://www.dragondriving.co.uk/harnessforsale-56308.html

What are the string boots for, and how do they work?