Zenkai means "the power you gain from taking high damage".
To get a zenkai, you must get knocked out or killed. Being killed gives a bigger zenkai. The stronger the person is who gave you the zenkai by knocking you out or killing you, the more power you will gain from the zenkai.
Zenkai Facts[]
The more often you zenkai the smaller a zenkai you will get. Zenkai needs time to recharge to maximum and each time you zenkai the time to recharge increases.
The power of the person causing you to have the zenkai is the most important factor. To gain any worthwhile power from zenkai the person must be stronger than you.
Race Zenkai Gains[]
Each Race gets a different Zenkai gain, whether bigger or smaller.
Humans - 1X Zenkai
Half Saiyans - 1.25X Zenkai
Legendary Saiyans - 3X Zenkai
Aliens - 1.4X Zenkai
Androids - 0X Zenkai
Bio-Android - 1.8X Zenkai
Demigods - 1.35X Zenkai
Demons - 0.9X Zenkai
Changelings/Icers - 0.95X Zenkai
Kais - 0.45X Zenkai
Makyos - 0.725X Zenkai
Majins - 1.8X Zenkai
Namekians - 0.45X Zenkai
Spirit Dolls - 1X Zenkai
Tsujins - 1X Zenkai
Saiyans - 3X Zenkai