Say I have $50k of equity in my margin account.
During the trading day:
I buy $40k of stock XYZ at 9:45
I sell $40k of stock XYZ at 10:00
I buy $40k of stock ZYX at 10:15
I sell $40k of stock ZYX at 10:30
I buy $40k of stock YYY at 10:45
I sell $40k of stock YYY at 11:00
I buy $40k of...