do_action( 'relevanssi_highlight_tokenize' )

This action hook runs before Relevanssi tokenizes the search query when highlighting search terms.

More information

One use for this action hooks is to disable the Relevanssi stemmer in the highlighting:

add_action( 'relevanssi_highlight_tokenize', 'rlv_remove_stemmer' );
function rlv_remove_stemmer() {
  remove_filter( 'relevanssi_stemmer', 'relevanssi_simple_english_stemmer' );

(I’m not sure how useful this is; I would rather suggest using the Snowball stemmer and enabling the “Expand highlights to cover full words” option.)