When picking a note or strumming a chord you have two choices; do you use an upstroke or a downstroke. It seems logical that a downbeat should be played with a downstroke, since this is usually the first note or chord of the song and it just feels right to hit that string confidently from above, much as a drummer would hit a snare drum. Using downstrokes for downbeats of course conversely means that upstrokes are good for upbeats. This is actually a very good system to connect the inherent rhythmic structure of the part you are playing to the physical feeling of playing it. Generally for most players, upstrokes are weaker than downstrokes, so it is a good idea to devote some practice time to strengthening them.