Mining centralisation, and what the pool share measures

A pool is a coordinator. Its members point their hash rate at the pool, the pool hands out work, and when a member finds a block the pool constructs the block template and claims the reward on behalf of the group. The pool's share of blocks is therefore observable: each pool writes a signature into the coinbase transaction of the blocks it finds, and counting those signatures gives the share of blocks each pool produced over a period.

The share is a measure of block construction, not of hash rate ownership. The hash rate behind a pool belongs to its members, who can point it elsewhere at any time. A pool with a large share is a large coordinator, not a large owner, and the distinction matters because the members are the ones who could leave. This is why the share is described as a concentration of coordination rather than a concentration of ownership.

The share also changes. Members move between pools in response to fees, payouts and reliability, and a pool that dominates one month may not dominate the next. A single share figure is a snapshot, and a series of them is a record of a moving distribution rather than a fixed structure.