Yes a trans can be flushed out. The WYNNS system hooks up to your transmission lines and uses 16 quarts of fluid; the natural force and flow of your trans fluid pushes out the old fluid and replaces it with the new. Trans fluid breaks down just like engine oil and needs to be changed. Now yamadooed is right about contaminates if you are having any issues at all the new fluid will definitely cause issues, the fluid contains additives that clean the system and when there are issues the cleaner tends to break down the clutch packs causing major issues.