This allows him to make his arms giant for punching or feet giant ... By assuming a stance where he crouches down and puts his fist to the ground, Luffy is able to speed up the flow of blood ...

Gum-Gum Giant is easily one of the most exciting ... displayed in the anime by giving a silhouette of Ace's Fire Fist behind Luffy, a symbol of Luffy carrying on his brother's will.