What are you even saying?
If the guy wants to see more definition in his arms, he needs to lose bodyfat. Unless he's a genetic freak that can lose BF and simultaneously gain muscle quickly (which he prob isn't since he needs to lose fat to see definition in his arms, no offense OP nearly no one is), he won't do that.
If your theory were true, then no bodybuilder would ever diet down for a contest, they all would be able to just cut up at the weight they start at.
You need to lose fat to see more definition, period.