Index full scan is the optimizer access path where the database reads the index from start to end, one block at a time.
It reads the index starting from the root block, navigating down using branch towards the left side, or right side if we are doing a descending full scan until it hits the leaf.
Once, it hits the leaf node, it read the entire bottom of the index, a block at a time using single block IO in sorted order. There is no multiblock IO used in this scan.
A detailed explanation of Index fast full scan can be found in my previous blog.