You might be interested in this visa category if you have a specific type of research fellowship, are appointed as an eligible senior academic or researcher at an HEI, are a research world leader and meet the Exceptional Talent criteria or you are at an early stage in your research career and meet the Exceptional Promise criteria. However, this a very complicated visa category and there are other requirements that you will also need to meet. You can find more detailed information about the Tier 1 ET visa category on the Home Office website.
You do not need to have a job offer or be sponsored by an employer for this type of visa application and you can undertake most types of work (there are a few restrictions) or be self-employed. The application process involves two stages where you need to first apply to the Home Office to be consdered for endorsement under the scheme (this is not a visa application). If you are granted endorsement, you can apply for a Tier 1 ET visa within 3 months. In most cases you will need to go home to apply for this type of visa but it might be possible for you to apply from inside the UK if you currently hold a Tier 5 Government Authorised Exchange visa. You can ask to be granted a visa under this category for between 1 and 5 years.