The partnership refines the written description, and signs it. Pairs attempt to find the function that fits the description. This may be an iterative process. When a pair proposes a function, the description writers may respond You've got it. That can't be the function because it doesn't satisfy a particular aspect of the description.
Additional rounds give the students practice matching functions to descriptions.
Prerequisites will vary depending on the functions used. It would probably be worth doing this exercise as a part of a general review of functions, and then again after the students have studied derivatives. Students who are stuck looking for a function should be instructed to sketch a graph that matches the description. Depending on the kinds of functions used, students could be asked to find the exact function or merely a generic version. The purpose of this exercise is to build the skills and intuition about functions that students need in order to, for example, choose an appropriate function form for a collection of data points.