Captain M,
I belive that the force exerted on the atomosphere by the exhausts gasses are neglible. That theory/roumor came about from a bad training film used in the RAF and certain other european airforces. It showed a man holding a garden hose, and when he put his hand into the waterstream it exerted an extra force. This cannot be transferred to jet engines because water is not compressible and air is.
Brgds
Doc