In contrast to a hard fork, a soft fork tightens existing rules rather than introducing new ones, which is what lets unupgraded nodes keep validating without splitting off.
A protocol upgrade that is backward-compatible, so nodes running older software can still follow the chain; they just don't see the new feature.
In contrast to a hard fork, a soft fork tightens existing rules rather than introducing new ones, which is what lets unupgraded nodes keep validating without splitting off.