The mechanism they are (probably) referring to is because of muscle damage acquired through exercise. When you have muscle soreness, it is due to the tiny little tears in the muscle fibers, which, when they heal, makes the muscle stronger.
Overall, it is a good thing, but the muscle damage does tend to pull fluids into the area in order to speed repair (at least in my experience). I have noticed when I lift heavy and have a lot of muscle soreness (like can't sit down kind of soreness), I tend to run 1-3 pounds "heavier" than normal (think days when I do large muscle groups and exercises like squats, lunges, deadlifts, etc...). But when the soreness goes away, so does the weight, and usually I'm right back to normal. In the long run, it's no big deal, because when the muscle heals, it is tighter, stronger and leaner. So I will take the extra fluid because I love lifting!
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