A constant force spring is a strip of flat material which has been wound to a given curvature so that in its relaxed condition it is in the form of a tightly wound coil or spiral.  A constant force is obtained when the outer end of the spring is extended tangent to the coiled body of the spring.  A constant torque is obtained when the outer end of the spring is attached to another spool and caused to wind in either the reverse or same direction as it is originally wound.  Torque is obtained only from the reverse-wound or larger spool. (Information from Spring Manufacturers Institute, Inc.)