Handling your horse's legs and hooves may be something you take for granted (it's easy!) or something you aspire to (you have a horse that doesn't give you confidence!)
Either way, it's all about safety, confidence and respect.
I see many horses that snap! their foot up the moment you go to touch their legs. It's a lot safer if you can teach your horse* to follow a specific request when you'd like to do something with their hoof. If your horse snaps their foot up, thinking you want it, or trying to avoid a pull on their fetlock feathers, then it's a lot more difficult to treat a wound, brush their legs or anything else!
Using the chestnuts as your specific request works well. It's a clear request that isn't easily confused with something else. I spend a lot of time around my horses' legs, ensuring that their manners are good, along with their hooves!
Young horses benefit too from this early education - here I am with young TB filly Cristel who is allowing me for the first time to stroke her legs - creating a solid foundation for the future.
(*included on my training DVDs)