According to Robert Maddock in Meatingplace (3/21/11), there are two types of electrical stunning, head only which affects brain function, and cardiac arrest stunning, which stops the heart.  Head only stunning is often used by very small processors and requires more training and labor than cardiac arrest.  Cardiac arrest causes less kicking and movement of the insensible hogs so carcasses are easier to handle during shackling and hoisting.

According to Temple Grandin (2015), head only stunning is reversible.  Pigs and sheep which are stunned with a head only stunner must be bled with a maximum interval of 30 seconds to prevent from regaining consciousness.  An interval of 10 to 17 seconds is recommended.  Head only stunning can be difficult in the case of shackling time and slow hoists taking up to 10 to 15 seconds prior to bleeding.