2.16 The grief which has not yet come may be avoided.
2.17 The cause of the avoidable is the superimposition of the external world onto the unseen world.
How much control do we have over events yet to come? According to Sutras 2.16 and 2.17, we can change future results by tweaking internal tenancies.
Goswami Kriyananda gave an analogy of how we can change our karma. First, understand that change takes time. If you were to sail a boat accrosse the Atlantic with ropes tied to hold the wheel at the helm in place, the boat would follow a natural couse until it reached the other side. If you have tried to tie something with ropes, you notice that there is a small amount of slack in the ropes. That small amount of slack is the will we have to change the direction of the boat. At first, the direction of the boat does not change much, but by the time you get accorsse the ocean, you may land in Africa instead of Europe.
In my practiced I have noticed that time not only changes the course of the boat, but the longer you adhear to yogic practice, the more the ropes loosen. After some time, some ropes begin to fray and eventually release thier grip on the helm of your life. The time it takes to effect change in your surroundings is inversely perpotional to how deeply a tendency is ingrained.
What does Sutra 2.17 mean by superimposing the world of the external on to the world of the unseen? The world of the unseen is what is inside us. Our ideals, memories, wishes, fears and expectations make us who we are. These are formed from impressions we pick up along the way from the external world. When we reverse the flow of impression from inside to outside, the world seems to change.