Why cannot. I can. I'm always on second gear when I'm below 40 kmh. Hehe. People who dunno how to play with gear better go back to driving school. Bodo one I tell you. That's why in Malaysia lots of jam at mountain/hills/bridge roads.

If it's an auto, no care which gear you put, say you're at 20kmh, your car will auto engage second gear.
Of course la if you no engage lower gear surely under power one. Ayam make sure my car have enough power when climbing hills. But sure it is fuel efficient for me.