Masking is the reason a mix can be perfectly balanced on every fader and still sound crowded. Two sounds that share a frequency region compete for the same place in your hearing, and the louder one wins. Not because it is louder overall, but because it is louder there. Turning the other one up does not fix it; it raises both the part and the problem.
The usual fix is a static cut: find the region where the synth and the vocal argue, and pull it out of the synth for the whole song. It works, and it costs you the synth everywhere the vocal is not singing. A wide cut held for three minutes to solve a problem that exists for forty seconds is a tax on the arrangement.
Un-masking is the dynamic version of that decision. The cut is made from the relationship between two tracks, in the moment they actually clash, and released the moment they do not. Nothing is removed while the vocal is silent, so the synth keeps its body between the lines and gets out of the way inside them.
The part that decides whether an un-masker is trustworthy is what it does when it is not carving. A tool that leaves a filter in circuit, or that re-synthesises the signal to work, is changing the sound at all times, including the moments it claims to be doing nothing. That is the difference between a claim you can hear and a claim you can measure.
Before you buy anything in this lane, check one thing: what the tool does when it decides to do nothing. Bypass it and null it against the source. If the difference is not silence, it is processing your track full-time and only claiming to be selective. Then check whether the cut is driven by the other track or only by a threshold on this one. A threshold is a de-esser with extra steps, not un-masking.
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