One example of dynamic valid-time partitioning would be to compute the average value of an attribute in a relation (say the salary attribute), for the previous year before the stop-time of each tuple. A technique which could be used to compute this query would be for each tuple, find all tuples valid in the previous year before the stop-time of the tuple in question, and combine these tuples into a set. Finally, compute the average of the salary attribute values in each set.
It may seem inappropriate to use valid-time elements instead of intervals, however there is no reason to exclude valid-time elements. Perhaps the elements are the intervals during which the relation is constant.