After the current active character inside a Blazing Threshold or Scorching Threshold triggers a Pyro-related reaction, the Traveler will regain 5 Energy. This can occur once every 12s. Additionally, when a nearby party member triggers a Nightsoul Burst, the Traveler will regain 4 Energy.
It's probably the free event weapon of 5.6, which rarely have good passives. This one is just a bet...