If you are on the defending side of beacon massacre you don't revive 90% power back. The tournament is designed to revive 90% of the number of units you lose. Offense wise this is much easier to control because you can send 1000 warlocks and revive 900. but defending you dont choose the troops that die so say you lost 100 units in total. 90 archers and 10 griffins, well in the worst luck the random generating varaible of coding will select for you to revive 90 archers and you lost all 10 griffins. that's definitly a power loss but it was a 90% unit count revive.