Fatigue Damage Mechanic

https://splinterlands.com?p=battle&id=sl_4b92fadb26ced0dc72fe2dee6f035bfa&ref=kingfish12

Was doing my daily challenge using an Earth summoner. Love the unicorn because it has a huge amount of health and thought I had this win in the bag. Then I learned about fatigue.

The "Divine Healer" card is a great card because it restores health with the "Tank Heal" ability. The downside of this card is it does not have any attack. However, after 20 rounds my cards couldn't defeat the Healer and started receiving "fatigue" damage.

I did not know about this mechanic and am unsure of the rules for it (if certain cars have different amounts of stamina or if after 20 rounds all cards always receive damage or if this only takes place in a stalemate). Very interesting mechanic and useful for when battles won't end just would like to see an explanation on the specifics of it.