I understand your skepticism at first, I also thought it was impossible, since Instagram hid the chronological feed of followers a long time ago. In reality, the mechanism is simple: these services use algorithms that periodically capture the state of your friends list and compare it with previous data. I’ve been using recently-followed.com for a while now and can confirm that it really works. You don’t need to hack anything the service analyzes changes on its own and displays new followers in a convenient format. It’s much easier than trying to figure it out on your own. It’s a huge help when you want to know the truth about an account’s activity without any extra effort.