As you may know, chanting Amitabha's name or Medicine Buddha mantra, etc, may bring you some benefits. For example, according this site:http://namobuddhamy.com/en/sadhana.html
You will receive the following benefits:
"1. Amitabha Buddha will always be over him, day and night, to bless him with safety and peace as well as to protect him from harm that his adversaries may want to cause him; at the time of his passing on, he will be able to take rebirth in the Blissful Pure Land of the West at will.
2. He will be able to lower his chances of meeting with calamities, reduce his bad karma, increase his prosperity and happiness, and lengthen his life.
3. He will be able to liberate those who have passed away and help them to take rebirth in the Blissful Pure Land of the West.
So it seems that even with bad karma, you may reduce the chances of meeting with calamities (as an example). How can a Buddha help you, since even no Buddhas cannot avoid the law of Karma? Isnt that a kind of paradox?
If I chant Amitabha name a lot and with pure hearth and faith, and even so I get caught by a flood, does that means that my previous karma was SO bad that even a Buddha couldnt not help?
In fact, I dont understand very well how the karma can be cleanse by "other power".