When playing chords lasting for whole, half or quarter notes, downward strums are used. However, when strumming eighth notes, it is common to use alternate strumming, which is similar to alternate picking.
All the notes on the beat (the number part of the count) are downstrums, while the notes between the beats (the and (+) part of the count) are upstrums. Example 39 uses alternate strumming in the final bar. Remember to keep your wrist relaxed regardless of which direction you are strumming.