Trigger context changes on activation, not focus
Moving focus is how people look around a page. G107 says looking must not do anything. Arriving at a control with Tab or an arrow key cannot launch navigation, submit a form, open a window, or otherwise change context. Those belong to an explicit act, such as Enter, Space, a click, or a separate Go button. W3C lists this as a sufficient technique for 3.2.1 On Focus at Level A, and it is the only one on that list, which makes it the single documented route to a rule you do have to meet. It applies to every technology, not just HTML. The mirror image is documented too. Yanking focus away with script the moment a control receives it is W3C's recorded failure for the same rule, so a page can break 3.2.1 from either direction.
How we find it in an audit
Automated tools cannot read intent out of an event handler, so this is keyboard work. Our reviewers tab and arrow through every interactive element and watch for anything that moves on its own. A popup that fires, a page that jumps, focus that gets pulled somewhere else. Select elements, custom menus and tab strips are where it usually turns up.
How affected users experience it
Keyboard and screen reader users move focus constantly, because focus is how they read the page rather than how they choose. When arriving somewhere counts as choosing it, a select that navigates on arrow-down puts every option after the first out of reach. The page is gone before they get there. Every Tab press then becomes a gamble on where they will end up.
Passes vs. fails
Passes
The select changes nothing while options are browsed, and navigation happens only when the user presses Enter or the adjacent 'Go' button.
Fails
A 'jump to section' select navigates the instant an arrow key lands on a new option, so keyboard users can never reach the second entry.
Other ways to satisfy this rule
2 guides on this site are filed under 3.2.1 On Focus. W3C lists this one as sufficient for that rule on its own. Implement it correctly, in a way your readers' software actually supports, and the rule is met.
This guide is our interpretation of W3C technique G107: Using "activate" rather than "focus" as a trigger for changes of context. W3C publishes its techniques as guidance rather than as the standard, and says so on every one of them. The success criterion is what conformance is measured against, and a technique is one documented way to meet it.