Any minor chord can be interpreted as a subdominant in a major or minor tonality

Take any natural minor chord in the initial tonality, or minorize any major chord, and re-interpret it as a subdominant of the new tonality. Follow up with a cadence 6/4 chord, dominant, and the tonic.

You can use this technique to modulate into a major OR minor tonality.

This is based on the sensation that the subdominant gravitates better if it's minor, which sounds very natural even in the major tonality.

A minor 6-chord works really well, but it could be any other permutation.

To make this type of modulation more interesting, you can add a small third below the main tone of the minor chord (turning it into the si-re-fa-la type of 7-chord) or think of it as adding a large second at the top (making it a 5/6 chord). In that case, it becomes a second diminished 7-chord (or 5/6) in a minor tonality, or a second diminished (harmonic) 7-chord (or 5/6) in a major tonality. By "harmonic" I mean the subdominant with the lowered third.