Hey

The IITs have announced the one year preparatory course for SC/ST candidates who have not qualified to be allotted seats for the current year. The Preparatory Courses is a one year residential programmes and as the name suggests prepares the students to be eligible for the B.Tech or Dual Degree Courses at the IITs. Basically it is held to improve the intake of SC/ST students in the Btech and dual degree programs. Every year around 40 students are selected from non-qualified  candidates (according to ranking order in JEE)(According to your query you asked about applying for it, but the Counsil itself selects the students based on JEE ranks). PD students are also eligible to undergo this course. The preparatory course consists of two semesters (1 year) after which result will be declared.  Refer the below link for more information

https://engineering.careers360.com/news/iits-announce-preparatory-course-for-scst-candidates

Hope this helps

All the best !