This new opcode, proposed in a Bitcoin Improvement Proposal (BIP), aims to facilitate direct access to the taproot internal key, thereby offering a more efficient method for executing certain transactions. By replacing the OP_SUCCESS203 opcode, OP_INTERNALKEY is tailored for scripts that execute under the leaf version 0xc0, allowing for a more byte-efficient process in transactions that involve multiple parties or require additional conditions beyond the standard key path spend.

The rationale behind this proposal is to enable a smoother integration of script-based restrictions with the taproot internal key, which can lead to significant savings, notably 8 vBytes per transaction. This efficiency is crucial in scenarios such as hash-locked transactions where the internal key plays a pivotal role. The proposed opcode not only enhances transaction efficiency but also opens up new possibilities for secure, collective transaction agreements without the cumbersome overhead of control blocks typically associated with script path spending.

A reference implementation of OP_INTERNALKEY has been made available on GitHub (reference implementation), outlining how this addition could be seamlessly integrated into the current Bitcoin protocol via a soft fork. This approach ensures backward compatibility while addressing potential issues related to the transition from the OP_SUCCESS203 behavior. Although still in the drafting phase, the proposal underscores ongoing discussions within the Bitcoin development community regarding its deployment and the broader implications for transaction scripting under the taproot framework. The openness of the BIP document, under the 3-clause BSD license, invites further review and contributions, reflecting the collaborative spirit of Bitcoin's evolution.