They need to self-select, according to a set of criteria, including whether they can support 20-minute aircraft turnarounds.
The trial period is ten days long, but you can take advantage of GameFly's "Fast Return" option for speedier turnarounds.
In one of the great turnarounds for the season, Izzie's condition deepens her relationship with Alex Karev and they share a very poignant romance complete with a fairy tale wedding.