Each summer is divided up into three three-week periods.
You submit a preference sheet with how you rank desire for each program. Between freshman and sophomore year everyone will take ESET for one period. Another period you may take soaring, jump, space or unmanned aerial systems (this is where your preferences come in) and then the remaining period will most likely be vacation. The preference sheet doesn't guarantee you a program but they try to get everyone their first or second choice of program.
There are different programs for the other summers. If you miss jump then you have the opportunity to take it your senior year. If you miss soaring and you still want to fly then you have another opportunity during the powered flight program.