User blog comment:Guts!/Anime Depiction of Luffy's Gear Second/@comment-972068-20130528014001/@comment-5601098-20130528142151

"After the timeskip, it appears that Luffy has refined his control over this technique as he has now been shown capable of activating it by pumping blood in only one particular part of his body: the trigger works very fast as he is capable of activating Gear Second in an instant and does not even have to assume his usual stance to initiate it. Thanks to this new found control over his technique, Luffy now has the ability to select which body parts Gear Second will be actived in instead of having to activate it in his entire body; though the anime depicts this as a particular body part undergoing the pumping motion, such as his arm. To perform any actions with this Gear, Luffy can now initiate Gear Second in the part of his body that he wishes to use and deactivates it once he has accomplished the task. Thus Luffy is now able to use Gear Second for his usual fighting style without using it on his entire body."

From the Gomu Gomu no mi page.