dbx's feedback suppression technology analyzes the audio that is fed through and looks for resonant frequencies that ring on longer and/or have higher average level than the other frequencies present. Once the algorithm has found a resonant frequency, it applies a digital notch filter to reduce the amplitude of the frequency to prevent feedback.
All you have to do is replace the words "reduce" and "prevent" in the previous paragraph with "increase" and "encourage" to see how the dbx algorithm could be inverted to actually create feedback.
It's not clear to me exactly how this works with in-ear monitors, but one way to do that would be that the notch filter, when in boost mode (instead of cut mode as used in dbx feedback elimination), could easily be made to self-oscillate and therefore actually synthesize the feedback frequency, meaning you can get feedback that naturally follows the guitar signal without needing any kind of acoustic signal path.
From their marketing material:
The DigiTech FreqOut pedal uses pitch detection technology to constantly monitor the harmonic content of your guitar signal.
If the guitar signal completely dies away, the feedback stops. Note that guitar signals persist on the electrical level longer than they are audible in some situations. So you might not be able to hear the guitar even if there's still enough level left for the FreqOut to keep operating. You just have to hold the notes on the guitar as long as possible.
When bending on the guitar, the strings move across the surface of the fret, so just bending strings makes them vibrate a little and maintains the note. In fact, one way to sustain a note on the guitar is to use vibrato, because the constant rubbing of the string by the fret acts like a very quiet violin bow to keep the note going. With the FreqOut, this vibrato sustain would be enhanced.
Regarding the Boss FB-2, that seems to require an amp to really create feedback. Their marketing says:
Advanced Feedback function provides smooth and natural amp feedback
So it's almost certainly boosting frequencies in a way similar to the FreqOut, but without the semi-synthesis aspect of it.