If you write a paper whose audience includes readers who are not particularly familiar with the subject you researched, a funnel introduction can help draw them in by starting with information they understand and care about. The funnel technique provides a smooth transition from something most people can relate to, such as the legends of King Arthur, to a specialized subtopic, thesis or conclusion that may be too obscure for the average reader, like a certain Irish manuscript's treatment of Lancelot.
In theory, you can start with as broad an introductory sentence as you'd like. However, if you start too broadly, it will take much too long to transition smoothly to your focused thesis, and the extra length will provide no additional benefit. Start with a sentence that introduces the field of study to which your thesis is relevant.
The funnel technique is particularly appropriate if the subject of your paper has multiple components, such as the effects of network architecture on MMORPG performance. In these cases, you can start with a sentence that introduces one component, such as networking, and then add one more component with each sentence. In this example, you could write a sentence defining networking, then describe what network architecture is, then how network architecture affects software performance, and end with your thesis about how it affects MMORPG performance specifically.
The funnel technique allows you to choose the angle of approach that will most successfully make your thesis relevant to your readers. Thinking in terms of the component-adding approach described above, you can choose the first component based on what your readers will find most familiar or care about the most. Starting with network architecture might be most relevant to network technicians, but if your audience is gamers, you could start with a sentence about MMORPGs and then slowly introduce the technical background that makes smooth gameplay possible.