I once decided to try and do an impartial test (being cynical about the benefits of magnets) so put them on a stiff old horse that wasn't mine and I didn't care much for one way or the other, to see if without wishful thinking, I could see a change. I THINK he moved happier, certainly trotted more, but the thing I did notice was his windgalls went!
Made a little difference. I put cool boots straight on after working hard or if there is any swelling and that seems to reduce the size of them more than the magnets did
My OH used magnetic boots last year when her Newfie had some problems with her rear legs, including windgalls. I was sceptical (as an engineer), but she now swears by them and they certainly seem to work.
Magnets are supposed to improve circulation, and if this is the case, then it is likely that the increased circulation will help drain / disperse the fluid build up in the windgall.
Alternatively the additional circulation may allow the horse to move around more, which would have a similar effect on the windgalls.
We tried magnetic boots on our ID/TB that had navicular syndrome when all else seemed to have failed. I was convinced within a few days of using them that they put him in greater pain and made him lamer. Whether the magnetic effect, by increasing the circulation, caused heat and swelling to the inflamed structures in the hoof I dont know, but I discontinued using them.
I could almost see the pain on his face after just one night of wearing them. Just had a really bad feeling about them.
He was much happier on 'bute twice times a day.
I can certainly see how they might be effective on windgalls though on that basis.