Rhythm playing position.

To get things going here's something I posted to the All About Jazz board:

To play Freddie Green style properly, try strumming like the old rhythm guitar players. If you watch any old clips (another trick I learned from dancing...) the rhythm guys played back and forth: Chunk 1 and 3 over the neck pickup and 2 and 4 more toward the bridge and snap your wrist slightly. All four beats should be stacato, with a slight accent on 2 and 4. It should NOT sound like, "boom-chick-boom-chick" unless your playing 20's or early 30's hot jazz. Swing should sound like "chunk-chunk-chunk-chunk".

Also, the beat should push slightly, and never be behind the beat. That's straight ahead jazz - not swing.

Give a listen:
2 beat: 20's-early 30's
4 beat: late 30's-40's