this pic is related with my previous pics "Luffy gear 4th" and "Luffy gear 4th Akuma Hanabi". here's the [link] and [link]

This is a simple explanation How my gear 4th version works :

Luffy's body's so dense in this form, his body looks overflowing. look at first panel(from right to left).

Since he's rubber, this overflowing mass can be pumped into any parts of his body. I'll take right hand as example. look at 2nd panel

Now his arm hold a huge ammount of mass and energy, It's REALLY heavy and the energy need to be transfered. So it exploded, but since Luffy's rubber, the explosion is mutated into a strong stretching fist. look at the last panel

SIDE EFFECTS :
- numb effecst for a few seconds(post attack effect)
- slow movement
- bad reflexes
- most attacks must be done in stationery position to get the optimal damage and accuracy