This explanation was posted on
www.groups.yahoo.com/group/DaytradersUnite/
Basically the daytrading rules came out of the Atlanta massacre when
the "crazed daytrader" who had lost his money shot up two daytrader
shops. A senate committee held hearings and so crying wives of men
who had lost the family "nest egg" daytrading "testified" mainly by
sobbing in front of the committee. Congress didn't pass a law but
the NYSE and the NASD proposed the PDT rules we all know and hate
in response to the aforementioned. If you read the letters of
comment it is clear that they all knew the rules were flawed but
they passed them as proposed.
It is interesting that while the rationale was reduction of economic
risk to the system they in fact turned most active traders loose with
twice the buying power they had before. (Most daytraders have more
than $25K) The remaining restrictions have no clear impact of
system risk. Why the magic number of four trades in a week? Why
the misinterpretation of free riding? (It used to apply only to
trading without depositing any cash, i.e. you make profit in the
three days to settlement and sell taking profit but the check never
shows up. That is free riding!)
Interestingly it seemed to favor the interests of the daytrading
firms themselves (by giving their clients 4 to 1 margin) and the large
institutions by keeping some of the small guys out of the way. The
losers in this were the online brokerages which had most of the
under $25K clients who trader somewhat actively compared to
traditional full service firms.