When you accumulate 5 stacks of Merciless Resolve or Relentless Focus and leave a battle, the stacks are saved permanently, which allows you to start a next battle using these skills.
The current mechanics of these skills create a huge advantage for gankers in PvP.
Bound Armaments is a similar skill, but it has a 10 second timer for using stacks.
Molten Whip is a slightly less similar skill, but still has a timer.
and so on