It is optional because he can control said haki. And yes, it's a different type of haki, didn't you read that fight? Luffy fuses his df with the haki, that's why it can stretch. Since the reveal of G4 Luffy used it on his legs and is extremely important, since it makes him able to move at high speeds through the air. The panel you showed is Luffy transforming his upper torso (without blowing air, meaning he can do it at will) because it's what Dorfy was binding with his strings.
Follows the same logic with one arm or even his nuts.
If you can't explain why he wouldn't be able to isolate a body part, then you lost this, since he already was able to do it with his legs as he wanted (and I repeat, the legs are as important as the other body parts in the G4 transformation).