When I first started pony club, one of the things we talked about was jumping, and cross country and stuff, and that came up in the conversations, and we were told that a lot of people actually use crisco which I thought was quite funny. I do remember watching the rolex one year, and one of the first jump combos was 3 huge I believe plastic barrels laying on their sides as bounces, and every horse that jumped them did slide both the front and back legs up and over the barrels which gave me a really good idea of how useful that stuff really is. Made it seem a lot less weird that they used it.
It is a form of grease. It is used for Mid to Upper Levels - what it is meant for is when and if the horse hits one of those unforgiving fences, they slick and slide over the obstible, instead of the obsticle gashing the skin and legs up.