Relative present tense is a relative tense that refers to a time that is simultaneous with some contextually determined temporal reference point.
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.