A proof-of-work network aims to produce blocks at a steady interval, yet the computing power directed at it varies constantly. Difficulty adjustment is the feedback loop that reconciles the two.
Block time is a target, not a guarantee
Miners search for a value producing a hash below a threshold. Because the search is random, block discovery is a probabilistic process with an average rather than a fixed interval.
Individual gaps between blocks vary widely. Long waits and rapid successions both occur naturally without anything unusual happening.
What the protocol controls is the average, and it does so by moving the threshold rather than by changing anything about the search itself.
The threshold moves against total hash power
If blocks have been arriving faster than intended over the measurement window, the threshold is lowered, making valid hashes rarer and the search harder.
If they have been arriving slowly, the threshold is raised, and finding a valid hash becomes correspondingly easier.
The adjustment is calculated from the timestamps of the preceding blocks, so it is derived from the chain itself and requires no external input.
Adjustment lags the change it responds to
Retargeting happens at intervals rather than continuously, so a sudden departure of mining capacity leaves the network running at a difficulty calibrated for capacity that has left.
Blocks then arrive slowly until the next adjustment, and confirmation times lengthen accordingly for everyone using the network.
The reverse happens when capacity arrives quickly, producing faster blocks until the retarget catches up and restores the intended interval.
Difficulty ties mining economics to price
Revenue per unit of computing power falls as difficulty rises, because the same hardware wins a smaller share of a fixed block reward.
When the asset's price rises, mining becomes more profitable, capacity is added, and difficulty rises until margins are compressed again.
The loop means mining tends toward a margin determined by electricity cost and hardware efficiency, regardless of what the asset is worth.
The mechanism also stabilises issuance
Because new units are created per block, holding the block interval steady holds the issuance schedule steady.
Without adjustment, rising capacity would accelerate supply creation, and the predictable emission schedule that underpins the monetary design would not exist.
Difficulty adjustment is therefore doing monetary work as well as operational work, and it is the reason the supply schedule can be stated years in advance.