Please note that this is the Narutopedia's article on the general skill. If you are looking for the article on Zetsu's personal technique then you should head to Substitute Technique.
With this technique, one replaces their own body with a block of wood or some other object, the moment an attack lands. This creates an optical illusion, making the enemy think the attack was successful. From this, the user can use the lapse in the enemy's attention to attack or flee from the battlefield. Explosive tags can be attached to the replacement for an added surprise. It's a basic ninjutsu even taught at the Academy, but it's a useful art that can be applied in variety of situations.
This technique is sometimes referred to as the Empty Cicada Technique (空蝉の術, Utsusemi no Jutsu) in other stories (not to be confused with the Empty Cicada Shell Technique, which bears the exact same kanji). This name refers to the empty shell a cicada leaves after molting.