prd refraction

As part of ACTION-851, I'm looking at:

http://www.w3.org/2005/rules/wiki/Modify_noloop

and trying to understand how, as per the comments, refraction (PRD
section 4.2.4) prevents the loop from happening.  Can you walk me
through it?  It seems to me that once the rule fires once, a new rule
instance is available, and I don't see how that new instance is not
fireable.

    -- Sandro