Freeriding occurs when you buy and sell securities in a cash account without covering the initial purchase.
Example A
You have $3,000 in your settlement fund. You purchase a stock for $4,000. Later that day, you sell the stock for $4,500 without ever paying for the $4,000 purchase. In this instance you incur a freeride because you have funded the purchase of Stock X, in part, with proceeds from the sale of Stock X.
Example B
You have $3,000 in your settlement fund. You purchase Stock X for $3,000 and Stock Y for $1,000. Later that day, you sell Stock X shares you have purchased without bringing in additional cash. In this instance you incur a freeride since the total amount owed for purchases made that day ($4,000) exceeds the settled cash you had to begin the day and you sold one of the securities purchased that same day.
Penalty
Your account is restricted for 90 days. During this time, you must have settled funds available before you can buy anything.