Stretch is always comfortable to wear but mechanical stretch takes it to a higher level. Normally it is the fabric itself that is stretchable; in this case it is the weaving that makes the fabric stretchable.
The loop at the back of this jacket enables you to attach it to your jeans, which prevents the jacket from creeping up: no more exposed back when riding your bike.
The seams have been triple stitched. This means there are three separate seams stitched parallel to each other to provide extra strength and durability.