Legend injections for horses?

I have an 8 year old belgian warmblood gelding who is a competitive 1.20m show jumper. He recently had a neck/shoulder injury because of inflammation which is common in show jumpers. My trainer injects her 18 year old ex-grand prix horse with Legend and when the vet came out to give it to her horse on friday, she asked if maybe it would benefit my horse. The vet thought it would and so he gave him a shot of it and he was jumping really well today. I was just wondering what it is exactly and what it does/any information on it. My understanding it that it lubricates their joints to prevent arthritis. Arthritis is caused by the fluid in between their joints wearing down and then the joints/bones grinding down on eachother so my belief was that Legend just adds a bit of "elbow grease" to their joints to make them feel better, jump well and prevent arthritis. Any information would be appreciated! :)