A proof of work network has no control over how many machines are mining it, yet blocks arrive at a steady average rate. Difficulty adjustment is the feedback loop that produces this.
Mining is a search with a moving threshold
Miners repeatedly hash a candidate block, looking for a result below a target value. The lower the target, the fewer results qualify and the harder the search becomes.
Nothing about the search can be optimised beyond doing it faster, so the expected time to find a block depends on the target and the total hash rate.
Difficulty is that target expressed in a readable form, and it is the only variable the protocol can control, since the amount of hardware pointed at the network is decided elsewhere.
Because each attempt is independent, adding machines does not make any individual search shorter. It simply means more attempts happen per second across the network as a whole.
The protocol measures elapsed time and corrects
At fixed intervals, the network compares how long the previous set of blocks took against how long they should have taken.
If they arrived too quickly, difficulty rises proportionally. If they took too long, it falls.
The calculation uses only data already in the chain, so every node reaches the same result independently without any coordination or announcement.
The size of each correction is also capped, which prevents a period of unusual luck or a manipulated timestamp from swinging difficulty to an extreme value in one step.
The loop stabilises issuance
New coins are created per block, so a stable block interval means a stable issuance rate.
Without adjustment, growing hash rate would accelerate block production and bring forward the entire supply schedule.
The adjustment is what makes a fixed supply cap meaningful, because it fixes the pace at which that supply is released.
Adjustment lags reality
Corrections happen at intervals rather than continuously, so a sudden change in hash rate is absorbed slowly.
When a large share of mining capacity disconnects, blocks come slowly and fees rise until the next adjustment restores the interval.
The reverse produces faster blocks and slightly early issuance until the correction catches up.
Difficulty and profitability chase each other
Higher prices make mining more profitable, which attracts hardware, which raises difficulty and reduces the reward per unit of hash rate.
Falling prices force the least efficient operations offline, which lowers difficulty and improves margins for whoever remains.
The result is a system that tends towards miners earning close to their cost of operation, with the adjustment mechanism doing the balancing.