In modern particle physics experiments, such as COMPASS, large data sets for multi-body final-states are collected. Such final states are usually studied using Partial-Waves Analyses (PWA), which employ the isobar model. In this model, intermediary states are assumed, which have to be described by a dynamic amplitude that has to be known beforehand and therefore is prone to possible model bias. To avoid this bias, we introduce the novel ``freed-isobar PWA'' method --- also known as ``model independent PWA'' --- that circumvents the need for fixed dynamic isobar amplitudes by replacing them by sets of step-like functions. Within this method, mathematical ambiguities --- ``zero modes'' --- may arise, which we show how to identify and resolve. We show an application of the freed-isobar method to a data set for diffractively produced three-pion final-states collected by COMPASS and will discuss selected partial waves within this analysis. In the end we give a short outlookon the possible application of the method to other data sets.