This excerpt taken from the ETFC 10-K filed Feb 28, 2008.
Know which orders are eligible
Our guarantee means that any qualifying stock or ETF order you place will be executed in 2 seconds or less and if it isnt, well give you a commission credit to use on a future qualifying order.
For an order to be eligible for the 2-Second Execution Guarantee, it must be:
There are certain conditions that make an order ineligible for the guarantee.