IUI QUESTIONS

so we have not tried iui yet I have only done letrozole.. my boyfriend's count is 5 million and not may are good swimmers I guess...im kinda confused about the semanalysis....but how does iui work?? do they pick out the good swimmers??? We really were frustrated with our first fertility dr..who was in the process of iui but cancelled, so we switched drs and basically started all over with just letrozole..which didn't work for the 3 months prior...we are at a stand still and wondering how the iui works and if we can be considered with his bad sperm??