I recently found this out by accident with my arena-partner. Not sure if it has been posted before as I can't search, but I'm pretty sure this is new to most of you.
You need a caster with a direct damage spell and a rogue. Here is what to do :
Rogue is stealthed near your target player.
Caster starts casting a direct damage spell. Let's say frostbolt.
When frost bolt hits, the rogue will sap at the exact same time. If done correctly, the target will take damage and get sapped.
Very useful for mage/rogue and similar setups against dualdps teams and druid + dps teams, allowing "free" spellcasts without the opponent being able to fight back. Also a good way to try and make frostbite procc, allowing the mage to open with a shatter.
This can be done with instant spells, but the timing might be harder. We did it by accident with Fire Blast.
NOTE : Do not use spells with a dot, such as Pyroblast. The dot will break sap.
My first attempt to contribute, plox don't flame me :x