KARMIC REACTION vs KARMIC INTENSITY
We must draw a distinction between the actual karmic reaction and its intensity. Even when an upāya (a prescribed remedy from a Vedic astrologer) succeeds it may only alter, not change, the specific karmic reaction that you will have to experience.
If in your prārabdha (fixed) karma it is written that a rock must fall onto your head, then a rock will fall onto your head. The only way you can get out of experiencing the rock's impact altogether is to have someone elect to take that karma from you, and have the rock fall on his or her head instead. If not, then the rock will almost certainly fall on you.
But it will make a tremendous difference to you if the rock weighs one gram, one kilo, or one ton. You may not even notice the impact of the one gram rock, and if you are wearing an upāya helmet, the one kilo rock is likely only to make your ears ring. Very few human skulls can however withstand the impact of a one-ton rock.
Given adequate available free will it is often possible to minimize a karma's intensity; it is far easier to change intensity than it is to change the karma itself.
Analyzing Karma : Fate or Free Will
Robert E Svoboda