Watch the Secondary Dominant online guitar lesson by Jason Loughlin from Focus On: Western Swing
The most common turnaround devices in western swing. It creates momentum and anticipation leading back to the tonic. The secondary dominant temporarily takes us out of key and creates the illusion of a new tonal center. You do this by treating any chord as a tonic and insert it's dominant chord as if that were the key you were in. You can also stack a number of secondary dominants in a row. This creates a cycle and prolongs the resolve to the tonic.