My guess for thief profession mechanic would be something similar to the Assassins Promise skill from GW1.
The skill marks a target for kill. if it dies u regain initiative + remove cooldowns + some other effect (boons/condition clear). So u are ready for the next fight as long as your health is still good. Most likely some of those effects can be changed with the new traits.
The name of the of the elite specialization would be Assassin.
just an idea