What does a Mechanical Stretch fabric mean?

A mechanical stretch fabric usually pertains to a fabric that is 100% Polyester but has a horizontal 2 way stretch to the fabric.  This allows the fabric to not have spandex but still be able to have a stretch feel to it.  The elasticity of the polyester and the criss cross processing of the production allows the fabric  to have its "spandex" feel.

What Benefits does Mechanical Stretch have?

Most Mechanical Stretch fabrics allow the fabric to breath better.  It also allows the fabric to be inherently Softer and Wrinkle Free.  Some fabrics with Mechanical stretch are Panne Velvet and Double Knit Scuba.  Both fabrics are well known for its ability to curve and hug the body.   Most Mechanical Stretch fabrics are also treatable with FR chemicals.  FR being Fire Retardant for apparel and Window Drapery needs by state law and or Fire Marshall requirements.