Relative tense is a tense that refers to a time in relation to a contextually determined temporal reference point, regardless of the latter’s temporal relation to the moment of utterance .
The -ing forms in certain subordinate clauses express relative present tense , as in the following:
Walking in both cases is understood as simultaneous with the time of the action of the main verb. This time may have different temporal relations with the moment of utterance .
Comrie, Bernard. 1985b.Tense. Cambridge, England: Cambridge University.