{feature 'search2'} renders its content while the flag is on for this visitor, and the part after its own {else} - or the @else@ half - while it is off. The flags are the setting $padFeatures, normally in _config/config.php, name => what decides it:

• TRUE or FALSE - on or off for everybody;
• a number from 0 to 1 - on for that share of the visitors, decided per visitor and the same on every visit: by a hash of the flag's name and the user's id, or for a guest a random id in a long-lived padFeatureId cookie;
• the name of a function - called with the user's row (NULL for a guest).

padFeatureOverride ( 'search2', TRUE ) fixes a flag for the rest of the request, whatever the setting says - for a test or a preview - and padFeature ( 'search2' ) asks from PHP. A name that is no flag is FALSE and, under the strict check, an error.

A share depends on the visitor, so that example is shown, not run.