That looks incredibly uncomfortable.
However, there are probably two ways to use this. One is for decline situps, where your knees would go over the bar at the end of the bench and your feet hook under the bar sticking out. Lay back until head touches then elevate towards feet.
The second is to lay the opposite way, and grab the bar at the end of the bench with your hands. Now instead of lifting your head/torso, you'll lift your legs/lower body.
In theory it would work "upper and lower" abs, but in actuality, the abs are one sheet of muscle. The activation is the same either way, as long as your shoulders are "curling" towards your hips, your abdominal muscle is being worked. As far as laying all the way back, there are a couple schools of thought. If you lay fully back or down, then your abs can relax for a moment. If you don't, then they remain under load. There are theories around time under tension producing better results for muscles.