Walking subcaucus

The walking subcaucus system is a method of proportional voting used in political party meetings to choose delegates to higher meetings. It is designed to ensure that people in the minority are able to elect delegates representing their views to the higher body, as opposed to plurality at-large voting, in which the majority may elect all the delegates from members of the majority viewpoint.
History.
The walking subcaucus system was designed in the late 1960s in the Minnesota DFL party, in reaction to the Eugene McCarthy–Hubert Humphrey contest, when it was noted that the previous rules allowed a bare majority to fill all the delegate spots with their supporters, thus denying any representation to minority viewpoints. It was a way of meeting the requirements of the Democratic Party Rules Reform Commission while retaining a caucus rather than a primary system.
The "walking subcaucus" system was planned so that minority viewpoints were elected to delegate spots in proportion to their support at the meeting. One of the principal designers was Leo Hurwicz, a 1968 McCarthy delegate and later Nobel Prize winner in Economics.
Use.
This process is typically used at a party meeting (such as a caucus) where a large group of people has to elect a smaller number of delegates to represent them at the next higher party meeting or convention. 
It basically means that the caucus meeting breaks into smaller sub-caucus meetings, and that rather than electing delegates at-large, each sub-caucus elects their own delegates from within their own group. The number of delegates elected from each subcaucus is determined by the size of that subcaucus.
It is also used in the Democratic caucuses of Iowa, calling the sub-caucuses "preference groups". It was highlighted in news stories in February 2016 when a coin-toss was used to assign the final delegates for some precincts when the number of participants dropped during the process and an insufficient number of participants existed to assign all the delegates.