VI+ sums the distance from each high to the previous low, VI- sums the distance from each low to the previous high, and both are normalised by true-range. Crossings of the two lines are read as trend changes.
It is conceptually close to the directional-movement-index with different arithmetic, and it produces similar signals with slightly different timing.
Its independent track record is thin, and its crossovers suffer the same whipsaw problem as every other two-line system in ranging markets. It is worth knowing mainly because it appears on many platforms, not because it solves anything the older indicators do not.
Related: directional-movement-index, adx, true-range, whipsaw, trend-following