The day trading buying power for an account is the buying power as of the close of the previous business day. The buying power formula is cash balance minus any margin requirements plus the amount that can be borrowed from stock held in the account. The largest open day trading position cannot exceed the start-of-day buying power. Any deposits, proceeds from closing overnight positions, or day trading profits cannot be used for day trading purposes until the following business day.
If the start- of- day margin equity of the account is below $2,000, the account holder can only use the day trading buying power once per day and cannot day trade using margin.
More information on day-trading requirements can be found here.