Needle up/down is a feature that allows you to select the position the needle stops in when you release the foot pedal or gas, i.e. if you select "needle down" when you take your foot off the "gas" the machine stops with the needle down in the fabric...always, no extra tap on the foot pedal.
When I got my Viking D1, that feature in combination with a pivot feature was absolutely amazing. With a pivot feature, when you take your foot off the gas, the presser foot lifts up about half way, making chain piecing even faster. Add the "needle down" to it and its like having an extra hand. Cool beans!
Those two features are "must haves" on my list if you were to buy a new machine.
Julie