I heard that using network testing purpose DDoS software through a proxy is?

Your question makes no sense. However, proxies play a big role in such attacks. This is because many proxies are run by the hackers and are used to take over the computer of the user. People who use these free proxies just invite the hackers in. In that way they can use other peoples computers to initiate attacks and other things.

Your question makes absolutely no sense at all. "I heard that using network testing purpose DDoS software through a proxy is as effective as refreshing the page a few times." is pure gibberish.

DDoS is Distributed Denial of Service -- an attack vector that uses "drone" systems on the Internet to take down the target system by flooding it with useless traffic. Since a DDoS requires the attacker to use other computer systems for illegal purposes without the consent of the owners it is a criminal act even for testing purposes unless you own all of the systems involved in the test or have the explicit consent of all of the systems' owners.