The default test is a linear test. Basically it downloads a known block size from the chosen server and calculates the speed based on the time it took. This is just like using a stopwatch to time how long it takes to download a file and using a calculator to calculate the throughput. E.g. if a 10MB file takes 20 seconds, then 10 / 20 * 8 = 4Mbps. TestMy will also return 4Mbps if the 10MB block took 20 seconds to complete.
A real world analogy would be like drawing water from a tap with a hose. Based on how long it takes to fill a bucket, you can work out the flow rate. E.g. if it takes a minute to fill a 10 litre bucket, the flow rate is 10 litres per minute.
The multithread test is quite different. The server splits the block up into multiple pieces and your browser downloads multiple pieces simultaneously. This process repeats until all the pieces have downloaded. For example, when I start a 50MB Multithread test, it says "Testing your connection with 46 x 1109 kB elements simultaneously...", so if my browser allows 10 simultaneous connections, it will download the first 10 elements, followed successive elements after each completes until all 46 have downloaded. Once all the elements have finished downloading, it calculates the speed based on how long it took to download the lot. Multithread tests tend to perform faster on shared networks, such as wireless, public Wi-Fi, cellular, etc.
For multithread, a real world analogy would be like carrying bottles of water from a water source, e.g. 10 at a time depending how many you can carry. Calculating the flow rate is much the same, e.g. if takes a minute to carry 20 x 1 litre bottles from A to B, that's 20 litres per minute.
