It's like this...

When your Pokemon grows a level, his stats get boosted, except Speed. Speed gets a below normal boost. Eg, you grow another level, and all of your stats are raised by 4 points, except Speed, which is raised 1 point.

I think that's how it works.

Anyway, I attached it to my Grovyle at level 27, and he's a level 50 Sceptile now, and Proteins, Zincs, etc. don't work on him. Hmm....I think the Macho Brace did that, but I'm not sure.