Many visitors assume that because service in Japan is often exceptional, a bigger tip must be an even bigger compliment. In practice the opposite reading is more common: offering extra money can be read as suggesting the staff member is not already being paid properly, or that their normal standard of care needed a financial incentive to appear. Neither reading is the compliment it would be elsewhere.
What happens if you try anyway
It is a genuinely well-known experience among travellers: leave coins on a restaurant table while walking out, and staff will sometimes jog after you down the street to return them, assuming you simply forgot your change.
Why the system does not need it
The concept of omotenashi — wholehearted hospitality given without expectation of anything beyond the listed price — is built into service-industry training and pay structure.
Menu and bill prices are final; there is no hidden gap that a tip is quietly expected to fill.
Staff are salaried or paid an hourly wage set by the employer, not subsidised by customer discretion the way US tipped wages work.
The one accepted exception: a small envelope of cash (kokorozuke) occasionally given to a ryokan (traditional inn) staff member or a very high-end private guide for exceptional, personalised attention over several days. This is a specific, pre-planned gesture, not a restaurant norm, and should be handed over discreetly in an envelope rather than left on a table or counter.