When a token launches, only part of the supply is liquid. The rest is allocated to founders, early investors, advisers, a treasury and future incentive programmes, and released over time on a vesting schedule. An unlock is a date on that schedule when a tranche becomes transferable. The schedule is usually published in the project's documentation before anyone buys.
The vocabulary is worth getting right. A cliff is a period during which nothing unlocks, ending with a single large release. Linear vesting releases a steady amount per block or per month after the cliff. Most schedules combine them: a one-year cliff followed by two or three years of linear release is a common shape. The result is that the first anniversary of a launch is frequently the largest single supply event in a token's life.
What an unlock does is make sale possible, not certain. Recipients may hold, may have hedged the position already, or may be contractually restricted beyond the on-chain schedule. But early investors who bought at a small fraction of the market price are profitable across a very wide range of outcomes, and a team funding operations from its allocation has a structural reason to sell regardless of price. Assuming nobody sells is the least likely of the available assumptions.
The right way to size an unlock is against liquidity rather than against market capitalisation. A tranche worth a few per cent of capitalisation is trivial for a large asset with deep books and can be several days of total volume for a small one. The comparison that matters is tranche size against realistic daily volume and against the depth of the order book within a tolerable price range. Where that ratio is large, the market has a genuine absorption problem on a known date.
Because the dates are public, the market anticipates them, which changes the observable pattern. Weakness often arrives in the days before an unlock as positions are adjusted, and the day itself can be unremarkable. This does not mean the unlock had no effect; it means the effect arrived earlier. Trackers that list forthcoming unlocks by size and date make this straightforward to follow.
Two things worth checking beyond the calendar. Schedules can be changed by governance, and a proposal to accelerate or extend vesting is material information. And unlocked does not mean sold: on-chain, the movement of a tranche from a vesting contract to an exchange deposit address is the observable event, and it is a more informative signal than the unlock date itself.