Back ages ago when I worked with standardbreds we would use the porter wraps under brace bandages for horses that “hit” at speed, which is what I thought you were referring to in the first post. Similar to how the eventers do it. That was when I worked for Garry Martin of Jadestone Farms - we raced at Northfields in Cleveland.
When a horse runs down it burns the underneath of the fetlock joint where it comes in contact with the track surface, not the pastern. I wish I could find a picture, but imagine cupping your hand right under the fetlock, above the pastern and you’re in the right area. I use the term burns because it is from the abrasion of the track surface, similar to a brush burn. A minor rundown will take the hair off, a more serious one will take some skin, then you can have an awful rundown where they affect the underlying structures and it looks like hamburger.
As for the 3Ms (yes, I do often make my own with 2-4 pieces of duct tape as well - its cheaper! lol), they can either be layered into the bandage or placed on the outside. Some people prefer them outside, others prefer them on the inside (I think because the end result is “prettier”). Personally I usually put them on the outside as I feel they are more effective.
Horses run down most often in the back legs but do run down up front as well. I have found that horses who have problems are more apt to run down up front than sound ones. For me, a run down in a front leg means I need to take a look at the horse and figure out whats wrong, unless it was because of racing on a lightning fast track and lots of horses ran down over the track that particular day/night.
These are the patches I was referring to:
http://www.bigdweb.com/FELT-RUNDOWN-PATCH-12PK/productinfo/1836J/
You can also buy a roll of the moleskin felt and cut your own, which is what I do when needed (cheaper and you can customize the shape).
http://www.bigdweb.com/MOLE-SKIN-FELT-ROLL/productinfo/6939/
And while I am posting links, here is a link for pre-made hock patches.
http://www.bigdweb.com/FELT-HOCK-PATCH/productinfo/1834J/
One last link - this is what we spray on the moleskin patches so they stick to the skin/hair IF they arent held in place with a vetwrap (for instance a hock or pastern patch):
http://www.bigdweb.com/STIK-TITE-ADHESIVE-SPRAY-12-OZ/productinfo/6495/