Atom Smasher is a simpler version of Split the Atom. The only differences are the amount of somersaults betweeen steps. Before, the added somersaults made the tricks much more difficult, however modern yoyos have made those extra steps a non factor.
Start with the Split Bottom Mount, perform and underpass with the non throw hand. Now your non throw hand index finger should be behind the yoyo, push the two front strings forward with that index finger.
Perform an underpass with the throw hand index finger, now should should be in front of the yoyo. Flip the whole thing over for a somersault, dismount and bind return.
Tricks to know before Atom Smasher: