In an engine, specifically those within cars and motorcycles, it is the combustion/explosion that causes the piston to move, and consequently, the car to move as well. [i]Locomotive Breath's[/i] punches work in a very similar way, in that when contact is made, an explosion with a blast radius of about 3ft occurs. The damage caused by one explosive punch is rather minimal, and non-lethal, but a fast series of blows could easily maim or kill a foe. 

In regards to Masa's defensiveness, his guardian will always attempt to prevent any immediate damage. For example, if someone fires a bullet at Masa, the guardian will summon automatically to punch the bullet, deflecting it and saving Masa. Additionally, Locomotive Breath is ethereal, meaning that its form can phase around Masa's body, in essence giving Masa the strength of his guardian. So Masa isn't entirely defenseless.