Watch the Using Trills online guitar lesson by Dave Celentano from Classical Concepts for Rock Guitar
The backing track features one chord (Am) and uses the horse gallop rhythm (think Heart's "Barracuda" or almost any Iron Maiden song). The harmonic minor scale is unique in that there are three different pairs of notes that are a half-step apart. In the key of A the notes of harmonic minor are: A, B, C, D, E, F, G#. See if you can spot the notes separated by a half-step. For the exercise, use the index and middle fingers to perform the trills.