Precasting is the term for exploiting a glitch which allows players to use certain skills in a duel before it has officially started.
Precasting is only possible with self-targeted skills such as Electric Shock which can be cast without a target. In a duel, if you cast one of these spells while your opponent is within it's range, they will take damage, even if the countdown is still going.
This is widely seen as unfair, mainly because someone such as an Elementor with a full INT build can win the duel before it has even begun.