I read an article in Popular Science about this. The drug doesn't weaken the memory of an event, persay, but rather it blocks the effects of stress hormones. So, instead of actually forgetting traumatic events altogether, it just deadens the reactions you get while recalling said events.

Direct quoting from the Popular Science on how it works:

1) Trauma triggers the amygdala to release stress hormones, which enhance memory formation in the brain.
2) Memories of the trauma are first stored in the hippocampus. Then a chemical reaction encodes them into neurons in the cerebral cortex, cementing them into long-term storage.
3) When a victim recalls the trauma, the memory transfers back to the hippocampus, where it can trigger the release of more stress hormones.
4) Propranolol blocks the effects of the hormones and softens the victim's perception of the trauma. The brain re-stores the newly edited memory.

/quote

So, in cases where people recall an event and start having severe reactions to it, id est they start hyperventalating, shaking, crying, panic...Propranolol weakens the emotional attatchment of the memory itself. So you'll still remember the event clearly, but you'll be able to recall it without all of the negative emotional side-effects you would have had before.

In fact, the link mentions that as well.


Personally, I'm not sure I agree with the use of the drug. I can see using it for people whose traumatic events effect their every-day lives, however.