What it changes
By default, the + on a search result adds the food to your plate so you can review and combine before logging. If most of your logging is the same foods at the same servings, you can skip that step: with Quick add from search turned on, tapping + logs the food immediately, at the serving shown on the row.
An Undo toast appears after each quick log, so a mis-tap costs one tap to reverse.
Turn it on
Go to Profile → Quick logging and enable Quick add from search. The setting only changes what the + button does — opening a food's detail to adjust the amount first still works exactly the same, and so does building a plate from the detail view.
Where it applies
Quick logging works across your search results, including your saved meals — a one-tap way to log a whole breakfast you've saved. The serving used is the one remembered from the last time you logged that food.