BIP 9, detailed in BIP-9, introduced the VersionBits scheme. Each block header contains bits that miners set to signal readiness for a proposed soft fork. If enough miners indicate support (meeting a defined threshold within a specific period), the soft fork automatically activates.

This process streamlined Bitcoin’s upgrade mechanism, making it more predictable and transparent. Without it, upgrades often relied on less-structured methods or required direct user consensus via other signals. BIP 9’s approach helps align the network on when to trigger new features while minimizing the risk of chain splits, although it sometimes leads to debates about miner power versus user node sovereignty.