return plumbing question

[lockrookie]

If you look at my return line on my 50g build you will see near the overflow a black capped on a t. It has 2 lines on it that I hookbairline hose to and feed to the over flow when pump turns off air enters the return line so it won't syphon the whole tank. Since my return line enters below waterline

If you want pics I can take some tonight for you.and post