Reduces the holder's Speed by 50%. In-game, this item doubles the EVs gained by the holder from any battle.
Doubles any effort values the Pokémon holding it gets, but halves its Speed in battle. Stacks with Pokérus.
Macho Brace is an EV-enhancing item, which means that it does this.
EV-enhancing items are items which enhance the EVs gained when a Pokémon is defeated. All of them halve the Speed of the Pokémon holding them in battle while they are being held.
These items' effects are doubled by the effect of Pokérus. For example, if a Chikorita with Pokérus that holds a Power Lens defeats a Haunter, that Chikorita would gain 12 EV points in Special Attack. Two would be from defeating the Haunter, four would come from the Power Lens, and the remaining six would be from the Pokérus's doubling effect. However, a Pokémon holding the Exp. Share will not gain any of these boosts, and unless already infected with Pokérus, will only receive the base EV yield of the defeated Pokémon.
Source(s) Experience, Smogon, and Bulbapedia