The measuring process is simple. After you start the test, you browser downloads a test file of about 100kB. If this file is downloaded in period lasts more than 5 seconds, the test is successfully finished and is saved to the statistics.
In other case, a new test file is generated by the server, so the download lasts more than 5 seconds. The maximal size of this file is 5000kB and the test results are valid also in case, that the downloading was finished in shorter time than 5 seconds.
The upload test is similar to the download one. The server uses data, you just downloaded while testing the download. The limit of the file, your browser uploads, is 1500kB.
This test should measure the response time (so-called ping) from the server to your computer. The test is unfortunately not very precise because of the used technology.
The principle of this test is measuring the downloading time of one file, but more times. After it, the best and the worst times are compared. If the file is downloaded in 2 sec (fastest) and in 3 sec (worst), that the stability is 66,7 percent.