GEAR SECOND

The first time luffy used Gear second was against Blueno.
DESCRIPTION:
This technique involves Luffy speeding up the blood flow in all or selected body parts, in order to provide them with more oxygen and nutrients thus making him much faster and stronger. The technique uses even more oxygen and energy, using up more food and causing Luffy to pant heavily. The increase in his metabolism causes him to be hungrier than usual or even have a paralysis type effect after prolonged use of it, like in the second fight with Rob Lucci, where Luffy was only able to lie on the floor. Lucci, one of the first of Luffy's opponents to witness it, compares the process to doping. Doping is adding more red blood cells to the body part/s, which in turn allows for greater delivery of oxygen and nutrients to the body part/s. In Luffy's case, his organs and blood vessels are made of rubber allowing them to expand and contract at greater rates without tearing. Essentially Luffy increases the blood flow by forcing more blood through his blood vessels thus increase his fighting ability. This mode's enhancements to Luffy's physiology give him the ability to use a new, stronger set of techniques which are generally the same as his old ones, but are so fast that even a skilled assassin would have a hard time actually seeing them. This is called Gear Two in the Viz Manga, Second Gear in the Official Simulcast subs, and Gear Second in the FUNimation dub.

Before the Timeskip

Prior to the timeskip, Luffy would initiate this technique by using his leg as a pump to increase the blood flow in all of his body parts. As a result, all of his body parts become stronger and faster. The technique used more oxygen and energy, using more food and caused him to pant heavily during use. He has been able to heavily damage some of the strongest individuals in the series whilst this technique was active, such as Magellan and Blackbeard. When this technique was in effect, his entire metabolic rate went so high that his sweat vaporized rapidly making his body look as if it were steaming and, in the anime, his entire skin turns slightly pink and shiny from the increased blood flow. He also says almost all of his attacks in English as opposed to Japanese.
At first, Luffy could only use Gear Second for a short period of time, because the accelerated speed at which nutrients are used up depletes his body rapidly. The mechanics behind this enhancement thus also mean reduced strength and agility after prolonged use, as pointed out by Lucci when Luffy's attacks start losing their speedy edge because with all his nutrients running low, having a faster blood flow would be useless as there are no nutrients to use. Due to this, as also pointed out by Rob Lucci, Gear Second hacks away at Luffy's life force dramatically, increasing a chance of having a much shorter lifespan because he is starving his body, though Luffy rebukes it, saying that it does not matter as long as he saves his friends. After Gear Second wears off, Luffy can recover quickly by replenishing his body's nutrients by eating meat. Later in the Impel Down Arc, Luffy demonstrated a more excessive use of his Gear Second ability without such exhausting costs like his fight with Rob Lucci, as well as being able to activate it at conveniently brief periods of time to save his stamina and strength reserves for longer fights. This is possibly because of Luffy's mastery over the form along with his desire to save Ace, and possibly the large amount of food he ate in Newkama Land at Impel Down.
While not in the anime and manga, in the 7th movie of the series which takes place before the Water 7 arc, Luffy briefly and unknowingly activated Gear Second during his fight against Ratchet, the antagonist of the film, when the sheer pressure of the machine Luffy is forced to ho
ld up unknowingly causes his legs to end up pumping his blood flow. He then defeated Ratchet with a Gomu Gomu no Jet Bazooka, although he thought that it was a regular Gomu Gomu no Bazooka.

After the Timeskip:

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. Although the anime still has his entire body undergoing Gear Second's effects, but it still only lasts very short periods of time. With this new mastery over Gear Second, combined with Busoshoku Haki, he can now easily destroy a Pacifista and take down a fishman as large as Wadatsumi with a single "Gomu Gomu no Jet Pistol"