Snape stuff

When Severus Snape was young he was a master potion-maker so he edited his potion's book and made-up his own spell 'SECSEMPRA'.In Harry's sixth year at Hogwarts he finds Snape's book and not knowing what the spell did he used it on Draco Malfoy then got in trouble with Snape. If you read the sixth book you will find out about it.