Technical explanation 

the X feed is a combination of needle feed, and drop feed. Drop feed is what is found on the conventional plain sewing machines, the material feeds when the needle is out of the work, the work stops feeding while the stitch is formed. Needle feed work is fed when the needle is in the work, this can help prevent the two plies from slipping.

X feed combines the two, feeding starts with the needle in the work as in needle feed (See A above), once the needle feed is complete the drop feed takes over (See B above) this helps close the stitch in the material.

The result   of all this also means that the machine can work with a much lighter tension with less strain on the thread.