Nijusan teaches two different types of motion. Some aiki clubs call these relationships omote and ura - and these names are probably as good as any. The problem with naming them, however, is that there seems to be a tendency to think of these relationships as positions or points in space - either in front or behind uke. Actually they are probably better thought of as paths through space, one beginning in front of uke and one beginning behind uke.
The techniques of nijusan are approximately evenly divided between the two paths. This, of course, does not mean that any given technique can't be done from the other entry - rather that the fundamental version that occurs most often in randori tends to be either one or the other of these types of motion. I will try to get some videos of these two motions uploaded soon so that I don't have to write out the thousand+ words that each pic is worth.
The cool thing about these paths is that they tie the techniques together better so that it doesn't so much appear that each technique is an individual thing that was delivered by God to osensei and which cannot change because they have been given an official and exotic Japanese name. Rather, the techniques become almost happenstance things that tend to happen at certain places on one path or the other.