When fabric is needled, the fibers intertwine with each other and this imparts strength to the fabric. This intertwining can actually look like loose knots. A stretch machine by pulling on the fabric in the machine direction can actually cause these knots to tighten and make the fabric stronger. Stretching in the machine direction will also cause the fibers to orient slightly more in the machine direction. Stretching will also increase overall line speed by the amount of stretch (% of stretch) applied. Stretching reduces the weight of the fabric by the % of stretch applied.
A stretch machine can be composed of all driven rolls or a combination of driven and undriven rolls. Since the fabric can be very strong once it is needled, stretch machines tend to have large motors and gearboxes and heavy rolls.