What happened
On 5 September 2022 Poolin, then among the five largest Bitcoin mining pools, paused all withdrawals, flash trades and internal transfers in Poolin Wallet, its custodial balance service, from 2 p.m. GMT, in order to preserve assets and stabilise liquidity. Founder Kevin Pan said user assets were safe. Poolin's own announcement is no longer online, so this is as reported at the time.
What happened next is on the chain and needs no press release. Through July and August 2022 Poolin found around 10 to 11 percent of blocks. In the weekly window beginning 12 September it found 4.6 percent, and 5.5 percent the week after (mempool.space pool data).
What it changes
It separates two businesses that look like one from the outside.
A pool assembles block templates, hands work to mining machines, collects the shares they return and finds blocks. That is one job. The other is holding money: accruing each participant's earnings and keeping them until they are withdrawn. Poolin froze the second while the first kept running normally.
The miners left anyway, and quickly. A pool does not own the hashrate credited to it. It rents it, and the lease ends whenever a customer edits a configuration file. Roughly half of a top-five pool's block share moved elsewhere inside a week, on a solvency scare rather than a technical failure.
What it does not change
Every block Poolin found was valid, before the freeze and after it. Custody of a balance sheet has no bearing on whether nodes accept a block.
It does not make concentration harmless either. Over 2022 to date, Foundry USA found 20.4 percent of blocks and AntPool 14.7 percent, and six pools account for roughly four fifths of them. A party finding most blocks could reverse its own recent spends, delay chosen transactions and orphan other miners' work. What it could not do is stated in the white paper itself, which says a majority of work does not open the system to arbitrary changes "such as creating value out of thin air or taking money that never belonged to the attacker" (section 11). Nodes decline invalid payments whoever mined them.
It also does not give anyone a clean measurement. Published "hashrate share" is share of blocks found, attributed by the tag a pool writes into its own coinbase. Nobody observes hashrate directly.
And the exodus did not make anyone whole. Repointing a machine takes minutes. Retrieving a balance from a company that has stopped paying does not.
Context
The precedent is GHash.io. In January 2014, at over 40 percent of blocks found, it published a commitment to stop accepting new mining facilities. It crossed half the network on a rolling basis that June anyway, and in July agreed to cap itself at 39.99 percent, asking miners to leave if it approached the line.
That episode set the pattern this one repeats from the other direction: the discipline on a large pool is that its hashrate is borrowed, and the lenders are watching.