There are natural causes of focus that can be used for other purposes. For example, when you are hungry, your mind easily achieves a singularity of focus, that I presume was used by our ancestors to focus on the one pursuit of satisfying hunger. Knowing full well that you have food stocked in the cabinet, you can trick your mind into thinking that whatever task you are working on is the means to your satiation. There is no complicated mental trick necessary. Simply become hungry and then sit down to work. The singularity or focus will come automatically. At some point you will actually need to get up to eat. But for a window of time, you will have this great focus.