How do countdown timers create urgency?

As an e-commerce business owner, I'm struggling to understand how countdown timers can actually drive customer behavior and increase conversions. I've seen them on various websites but want to know the psychological mechanisms behind their effectiveness, how to implement them strategically, and what makes a countdown timer compelling enough to motivate potential customers to make a purchase. I need concrete insights into turning these visual elements into powerful conversion tools that don't feel manipulative.