A sneak attack lets you take an extra 50hp off your opponent. To use a sneak attack, after you attack, shout sneak attack in block capitals. If it's not in capitals then it doesnt count. it has to have 3 exclemation marks at the end aswell. it would go something like this:

Kamehameha!!!
SNEAK ATTACK!!!
1 and 5

If they say their 2 numbers first it doesnt count and if they have an item that detects sneak attacks then don't even try it!