Akuma floats toward the oppenent and grabs them, the screen then goes blank and flashs are seen. Shun Goku Satsu is an Insantant Kill move.
Related to the Street Fighter gaming franchise. The name refers to the most lethal technique of the Ansatsuken fighting style taught by Goutetsu, a style practiced by Ryu and Ken (among others). It was first seen performed by Akuma (Gouki) on M. Bison (final boss) in SSF2T, when certain Arcade mode requirements were met. Subsequent users of this technique (or its variants) include Evil Ryu, Dark Sakura and Dan (though his is generally more self-destructive than useful as an attack except in Capcom vs SNK 2, where it is actually highly effective).
Several theories exist regarding what happens when the screen blanks out, following the short teleport-like move by the user.
One version has it that the souls of both victim and user are momentarily teleported to another realm where they are attacked by demons of their past, hence the inherent danger it purportedly places on the user.
A second theory suggests that the user momentarily enters a frenzied state, delivering rapid attacks equivalent to upwards of 15 Hadoukens (chi bursts) at point blank range. This is demonstrated particularly well in Street Fighter IV. In it, multiple purple blasts resembling Akuma's regular hadoukens flash between Akuma and his opponent when he performs this technique.
The shun goku satsu is also known by the English versions of its name such as the Raging Demon and Instant Hell Murder.