Set something running, close the tab, come back to the result. That is the whole idea, and the rate does not change because you were not watching.
The rules, exactly
| Rule | Value |
|---|---|
| Offline progress per return | up to 8 hours |
| Grace window treated as continuous play | 45 minutes |
| Rate while offline | identical to active play |
Why it is capped
An uncapped idle game rewards absence more than play, and eventually the only optimal strategy is not to open it. Capping the return keeps a session worth having while still meaning a night's sleep is never wasted.
What it does not do
Offline time trains the action you left running. It does not fight world bosses for you, trade on your behalf, or make decisions you did not make. Combat, bosses and the market are things you turn up for.