The boost value (Both positive & negative) is meant to change the trending (T-Score) score of the product. For the products to be sorted based on the boost score, the default sort order on the page should be set to trending.
Example: Consider the following case, where products with the Color - 'Yellow' tag are selected and a positive boost of 205 is applied.
For these products to be sorted based on the boost score, the page that contains these products should be sorted by 'Trending'. Please find the below screenshot for reference.