If you have not read the Satipattana Sutta yet I would advise you to read it. It is one of the Suttas in a collection of 20 Suttas that we have compiled and we can email it to you. The Satipattana Sutta has something like 35 different segments to it, 35 different instructions and at the end of every one of these 35 segments, the Buddha states a very similar statement. He talks about knowing that particular object such as the hindrance of aversion or desire, etc., knowing that it is impermanent, that it has its own conditioning that causes it to arise and to also pass away. If you can't actually know or understand that, then the Buddha states to simply establish mindfulness with the thought to the extent necessary just for knowledge and mindfulness.
Now, this particular knowledge, "the thought to the extent necessary," it is not totally clear what is actually meant by this "extent," but we can interpret it with the fact that everything is impermanent. To me, this is the easiest way to interpret this extent or understanding, the knowledge we want is to understand that all things are impermanent. You don't have to know the causes that brings something about, you don't have to know the causes that will make it go away, but it is going to go away, it will all pass away. It is all going to pass, it just is what it is, it is impermanent.
So, a lot of those stanzas at the end of each segment in the Satipattana Sutta refer to having the knowledge that basically everything is impermanent. Now when you truly have the understanding that things are impermanent, you start to let go. What is there to hold onto if things are going to disappear anyhow?
How do you understand this? Say I love this particular shirt, I hold onto it, I cherish it, even though I am attached to this object it is going to age, wear out, get holes, rot and return to the earth. Understanding impermanence is very helpful so that we are not so attached to things and we don't cause more Dukkha for ourselves.