It is executed the same as a regular Hadoken, albeit in midair. The user thrusts their hands and fires an airborne Hadoken, angled down.
The angle is usually about 45° downwards, and the speed at which the fireballs fall depends on the strength of the punch button pressed, with the weakest going slower, and the stronger falling faster. In Street Fighter IV, the angle of Hadoken also depends upon the punch button used.
The Zanku Hadoken is great for stopping close-range counters while in the air and assaulting grounded foes. However, in some more aerial oriented games, the Zanku Hadoken is not very ideal for air-to-air combat due to its angle.