You should...
You should see the Frenzy animation when your stack kills an enemy stack, and then after that you should see that you now have the +Attack increase mentioned in the description.
It at least works in my current HOMM3 Babies mod game, but I didn't change this part of the game, so it should work with your stock patched game as well.
If you kill more enemy stacks the value will continue to increase, too.
/C\/C\
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