The term Zenkai is generally used to describe the near-death boost a Saiyan warrior experiences when surviving possible life-threathening injuries. When a Saiyan received a Zenkai, their power usually soares to incredible new heights. The rate at which they increase, seems to heavily depend on the severity of the injury. A Saiyan will not get a Zenkai boost when they are killed, and subsequently brought back to life courtesy of the Dragonballs. Also, it was stated in the manga by Vegeta, that self inflicted damage will not result in a Zenkai.
 Exposure to new healing methods
Is is theorised by fans that when a Saiyan is exposed to a type of healing their bodies have never experienced before, their power increases more exponentially then it would when the Saiyan constantly uses the same method to heal. For example, Vegeta's power jumped from a level below Recoome's, to nearly matching Frieza's 1st form in brute strength after he took his first ever Senzu Bean. The same example works for Goku, who was able to fight evenly with Frieza after he was healed in a rejuvenation chamber for the first time in his life.
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