The macho brace doubles the EVs gained for all stats but it reduces only the speed in battle
Example taken from the DBs EV page:
A Starly normally provides 1 Speed EV, but with your Pokémon holding the Macho Brace, it will receive 2 Speed EVs.
So that means when you kill something that gives 2 hp evs then with the macho brace you would get 4 hp evs
and it is only in effect when your Pokemon is holding it
Hope I helped :)