Second Wind, called Chitty Chitty Yang Yang before being unlocked, is a passive ability unique to Yang Xiao Long in RWBY: Grimm Eclipse. It must be unlocked by reaching level 10 as Yang. This ability fully refills Yang's Ultimate pool when her aura is depleted, with a 30 second cooldown.
Survivor is a good pair for Second Wind, as it will allow Yang to spend more time with her aura depleted. While it may be useful to allow enemies to hit Yang to refill her ultimate, be aware of the cooldown and don't try to pull this too often.
Early Access Edit
During early access, Second Wind was called Relentless before being unlocked. It was unlocked by defeating 5 enemies while Yang's aura was depleted. Like the full release version, it completely refilled Yang's special when her aura was depleted, with a 30 second cool down.