The frequency of a test determines how much time passes between test runs from each node. Depending on your account settings, the test can run as frequently as every 5 minutes.
Whenever a node completes a test, a time stamp is placed on the data it has collected. At that point, the node checks the test's frequency setting and schedules the next time it will run the transaction.
When a test's frequency is altered, the node will not make any changes to the next scheduled test. In other words, the next scheduled test transaction will still use the old frequency one more time. At the completion of the next test run, the node will use the frequency changes when scheduling the next test transaction.