Physical mechanisms of explosive boiling of an ethanol-in-decane emulsion droplet are investigated by numerical simulation. Vapor bubble growth due to explosive boiling leads to secondary breakup of the droplet. The ethanol mass ratio is chosen in the regime of puffing (vapor ejection and partial breakup), which is generally likely to occur for a fuel spray in a combustor. Using both free and sessile droplet configurations and varying the initial number of boiling bubbles and the depth of the bubble formation in the dispersed sub-droplets, the droplet breakup and vapor mixing processes are discussed. For both free and sessile configurations, the basic dynamics of puffing is the same. When the bubble depth is deeper in the sub-droplet, the bubble growth is eventually larger. When the number of bubbles is more, puffing occurs several times, or the bubbles coalesce leading to larger bubble size, larger breakup and enhanced vapor mixing. In the sessile droplet configuration, the vapor ejection dynamics is essentially similar, but particularly when puffing is strong, the droplet bounces from the wall, first pushed toward the wall by the ejected vapor and then pulled from the wall due to the repelling and recoiling motion. Under such a condition, the initially attached droplet finally detaches from the wall and returns into the ambient gas. This phenomenon may transiently influence the mixture formation in a combustor when a fuel spray collides with the combustor wall. The present study serves as a first step toward a goal of quantitatively evaluating the puffing/microexplosion effect on a real fuel turbulent spray in internal combustion engines as to how large it can contribute to improving combustion performance, and insights have been obtained for future modeling which is expected to fill the gap between the droplet-scale and spray-scale knowledge.