what does luffy's "second gear" do?

Because his blood is being pumped faster, this causes his whole system to literally 'run' faster... If a normal human's blood is pumped faster and faster, their сердце would certainly explode, even if they were able to run at a higher rate than usual... In Luffy's case, Eiichiro Oda has exaggerated the benefits that running your blood faster, would give, but the idea is still the same.