Fifteen months after first introducing 24-hour trading for a select group of ETFs, TD Ameritrade Holding Corp. (NYSE:AMTD) announced Monday it's expanding its lineup of ETFs that can be traded 24 hours a day, five days a week.

In addition to the 14 original securities, the 10 new funds that can be traded overnight are:

• iShares Core U.S. Aggregate Bond ETF (AGG)
• iShares Core S&P Mid-Cap ETF (IJH)
• ProShares Short Dow30 (DOG)
• Vanguard Total Stock Market ETF (VTI)
• iShares MSCI EAFE ETF (EFA)
• Energy Select Sector SPDR Fund (XLE)
• iShares MSCI Australia ETF (EWA)
• Financial Select Sector SPDR Fund (XLF)
• iShares MSCI Japan ETF (EWJ)
• Technology Select Sector SPDR Fund (XLK)

According to the retail brokerage, 70 percent of clients interact with the firm’s research and education resources outside of regular trading hours. They also found that clients who trade during the 24/5 session are 10 times more active and have 3-4 times more assets than typical retail clients.

“What we’re finding with our clients over the last year, is the types of clients who trade during the 24/5 session happen to be some of our best clients. They’re the most engaged clients that we have,” said Ovi Montemayor, managing director, financial markets services.

Montemayor noted these ETFs were specifically selected because they are diverse, heavily traded and can be tightly correlated to another market to control the spread.

“In selecting these symbols, it’s critical that we are creating a very rich experience. In other words, they need deep liquidity, tight quotes, and it has to be a very accessible market just like during the day,” he told Benzinga. “One of the most important criteria behind the scenes encompasses that the securities that are trading 24/5 are tightly correlated to another market that has a robust price discovery process.”

Orders placed during extended hours overnight session (noted as EXTO), are valid through the end of one 24-hour trading day, from 8 p.m.- 8 p.m. ET. All EXTO orders are routed to the same alternative trading system (Pro Securities), and Virtu Financial Inc (NASDAQ:VIRT) is the primary liquidity provider. In addition, all 24/5 trades must be placed as limit orders.

In addition to requiring limit orders that control risk, Montemayor said the firm has a number of other risk controls in place to prevent the ETFs from deviating too far from their true market price.

Still, traders interested in participating in the 24/5 session should be aware of the added risks, such as lack of liquidity.

Montemayor also said the firm has observed that overnight volume is highly correlated to when there are news events. But with the introduction of 10 new ETFs to the 24/5 market, they are modeling for even more volume.

“It’s more of a function of if you have a store and you have three shelves full of stuff and everything else is empty, you tend to have less people at your store,” he said. “But when you have a full store and every shelf is full, there’s more options for clients to come in. So we think the broader offering is actually going to expand the engagement and we’ll have an uptick in overall use.”

TD Ameritrade's stock closed Friday's session at $52.11 per share.