It is not always desirable to leave a chord ringing once it has been played. In Blues it is common to cut the sound of a chord off as soon as it has been played, making the rhythm very crisp. The technique of cutting chords or notes shorter than their written value is called staccato. With chords contining open strings, staccato is usually achieved by placing the side of the right hand across all the strings immediately after strumming the chord. Staccato is indicated by placing a dot directly above or below the intended note.
This one uses staccato on the second and fourth bets of each bar. You may have heard this sound used in early Jazz songs.
Here's another one to practice, this time changing between major chords and 7th chords.