Although the degree of atherosclerosis (hardening/narrowing of the arteries) correlates with the risk of heat attack, the correlation isn't perfect. In fact, if you read my previous post on 20th century coronary heart disease trends in the U.K., you know that the frequency of heart attacks rose dramatically during the first half of the century, while the prevalence of severe atherosclerosis stayed the same or even declined.
If you accept the standard idea of how a heart attack occurs, first the coronary arteries become narrowed due to atherosclerosis. Then a clot forms, which lodges itself in a narrowed artery, blocking it and cutting off the blood supply to part of the heart muscle. The clot may be the result of a ruptured atherosclerotic plaque.
If you're unlucky, the loss of blood to your heart causes arrhythmia, or a loss of coordination of the heart muscle. This can cause it to pump blood inefficiently, sometimes resulting in death. Arrhythmias are estimated to account for about half of all heart attack deaths in the U.S. Sometimes they occur without a coronary blockage as well.
Omega-3 fatty acids seem to affect all three parts of the process: the atherosclerosis, the clot formation and the arrhythmia. Supplementing fish oil, even in the absence of reduced omega-6, may slow the progression of atherosclerosis according to a controlled trial.
Where omega-3 really shines is its ability to prevent clots and arrhythmias. In the DART and Lyon trials, the benefits of improving omega-6:3 balance appeared much more quickly than would be possible if it were acting by reversing atherosclerosis. This may have involved the blood-thinning properties of omega-3. The most dramatic effects were on sudden cardiac death, often the result of arrhythmia. Omega-3 fatty acids potently suppress arrhythmias in animal models.
You can have severely narrowed and calcified arteries, but if a clot never shows up, you may never actually have a heart attack. The modern industrial diet is extremely thrombotic (clot-promoting), probably in large part due to the combination of excessive omega-6 and insufficient omega-3. If the artery blockage doesn't cause an arrhythmia, the heart attack may not be fatal.
Omega-3 fats seem to prevent heart attacks on multiple levels.
The Lyon Diet-Heart Study: Background
The Lyon Diet-Heart Study
The Lyon Diet-Heart Study: Implications
Polyunsaturated Fat Intake: What About Humans?