A quick short cut you can use is toggling the little arrow next to the check box to create a short cut for turning on pre market data directly from the style drop down menu in the top blue bar.
Using pre market data (extended hours data) is critical in determining which stocks are gapping up, which stocks are gapping down, or where we should expect buying and selling to commence. Sometimes we see large moves in the stock market when the pre market high is broken, and on the flipside of the coin large moves lower when pre market low is broken in the regular trading hours session.
By using think or swim (tos) we have all of the tools at our finger tips to see all of this critical data to make better informed trading decisions.