to simplify...
PDT allows you to borrow intraday 4x your account...
$50K => $200K Notional
If you buy $100K @ 10AM, your buying power avail will be $100K
If you sell $101K @ 11AM, your buying power avail is $204K (profit)
if you sold $99K @ 11AM, your buying power avail is $196K...