What's this piece of gear for?

I'm sorry I can't find a pic on the net. On the front of the 'Ride World Wide' holiday brochure is a pic of a horse being ridden somewhere in South America, wearing a long strap that comes from the back of the saddle and down loosely round his hocks and up to the saddle the other side. There is a strap from the centre back of the saddle to the centre of the lower strap to hold it up to hock level. The whole thing looks like it could be stepped in very easily. This horse is pictured out on the range so I can hardly believe it is some kind of schooling aid, nor is it purely decoration as it looks well made. Does anyone know the bit of kit I am talking about, and what its for?