not far from my home, there are many steep roads. On bike, you can go uphill (i.e. you are allowed to, but coping with that is a a different point), but the road is closed for bicycles downhill. Motorized vehicles (also motorbikes) and pedestrians, can use it in both directions. Examples are e.g. road St 2186 from Streitberg to Oberfellendorf, or St 2185 from Behringersmühle to Gößweinstein.
How can that be tagged correctly?
This is only one of the proposed taging methods. As far as I know, there is no standard yet. Please see: http://wiki.openstreetmap.org/wiki/Proposed_features/Conditions_for_access_tags