In plain language
Changing a field's value (typing, selecting, checking) must not unexpectedly navigate or submit.
Who it affects
Keyboard and screen-reader users, and people who need predictable form behavior.
How to check
Change dropdowns, radios, checkboxes without pressing a button: the page must not jump elsewhere or submit unless it warned you first. Worked example — A community forum's discussion thread: type in a search box and stop typing (don't press Enter): if it auto-navigates to a results page after a certain number of characters, it fails.
Common misconception
A dropdown that navigates on selection (a “jump menu”) is a classic, well-intentioned convenience feature that actually breaks this rule, since changing a value shouldn't trigger navigation without warning. Real example: on a customer-support live-chat widget, the form submits itself automatically the moment the last field is filled — the user never chose to submit and may not be ready.