Mantra practice is often explained in terms of tradition or vibration. There is also a mechanical account, and it rests on how the mind holds verbal material moment to moment.
Inner speech uses a limited channel
Holding words in mind depends on a rehearsal process that repeats them internally, and that process has a narrow capacity that cannot run two verbal streams at full strength.
Anyone who has tried to read while someone talks nearby has felt the interference, which is why the effect is easy to demonstrate without any equipment.
The same limitation applies inwardly, so a mind occupied by continuous deliberate repetition has less capacity available for spontaneous verbal thought.
Rumination is largely verbal
The repetitive worry people describe as mental chatter is mostly propositional and language-shaped, arriving as sentences about a situation rather than as images or sensations.
Occupying the verbal channel therefore competes with rumination specifically, in a way that watching the breath does not, since breath awareness is not primarily linguistic.
This offers an explanation for a common report, which is that people who cannot settle with breath practice sometimes find repetition immediately easier.
Meaninglessness serves a purpose
Traditional mantras are frequently in a language the practitioner does not speak, which removes semantic content that would otherwise invite interpretation and association.
A phrase in one's own language pulls toward its meaning, and meaning generates further thought, which reintroduces exactly the activity the repetition was intended to displace.
Sound without semantics gives the rehearsal process something to hold that leads nowhere, which is a design feature rather than an accident of translation.
Rate and effort shape the result
Rapid, forceful repetition demands more effort and tends to produce a narrow, tense concentration that many people find effortful to sustain for long periods.
Slower repetition, allowed to become faint and irregular, occupies less capacity and is typically described as settling rather than as gripping.
Most instruction addresses this indirectly, telling students not to force the mantra, which is a practical way of pointing at the same tradeoff.
What the account does not claim
Describing a mechanism for reduced verbal thought says nothing about the meaning traditions attach to particular sounds, which is a separate question this account does not address.
It also does not imply that displacing rumination resolves what the rumination is about, since a quieter mind and a solved problem are different outcomes.
Persistent rumination that interferes with sleep, work or relationships is a recognized clinical concern, and evaluation by a licensed professional is the appropriate step there.