This is a variation of the guaranteed critical hit exploit and it allows you to preform 2 attacks at the same time with a melee weapon. Its the most deadly for a stealth spy but its useful for any class.
The steps to preform this exploit are:
- Melee Attack and press lastinv (q key) right away
- Press lastinv (q key) again to switch back to your melee weapon
- You should automatically attack about 1 second
- Make another melee attack at the same time for an instant double hit
Here is a video by Estryark showing the exploit in action.