IUI with trigger shot...really confused 🧐🤨

Nikki

So just had my infertility appt and my Dr wants to do an HSG to make sure the flood gates are open first. Then he wants me to do clomid again (4th month) and monitor my follicles under u/s. He said if the follicles look mature enough next Friday (CD 12) he’s going to give me the trigger shot and have me come in the next day for the <a href="https://glowing.com/glow-fertility-program">IUI</a>. I’m confused what the trigger shot actually does. Does it force ovulation earlier? Because I have been ovulating on day 17 for the past 4 months and that didn’t seem to mean anything to the Dr. If the follicles aren’t really ready that Friday he just tells me when to come back for another check?? So confused how this all works🤯🤯