Vipassana Bhavana

So now I fully understand Samatha Bhavana meditation and the goal of all meditative practice - thanks to some of you guys' helpful answers. However I fail to grasp how to practice the second type of meditation : *Vipassana* Bhavana. I've looked at most of the answers attempting to answer similar questions to mine here on this website, but didn't find them *that* helpful. I know of course what the position you're supposed to adopt is - most traditionally you'll do it sitting, the back straight preferably against a surface such as a wall, etc. But what is your *mind* focusing on during Vipassana ? Thank you