Some people had the same question about the Water Main's heal.... I don't understand what's so hard to understand...

Everybody's health has a number value. The Earth Main will shield and the Water Main will heal whoever has the lowest value.

A lvl 5million Susano'o Itachi can be dying with 10k health, while a lvl 1 real OG super rare Jiraiya is full health at 1k health. The earth main will still heal the Jiraiya.

Test it. I guarantee it.