I heard that by burying stretched stems when repotting, the buried stem actually begins to grow its own roots. Is this true?
yeah when you transplant bury it up to the bottom leaves and the stem below the soil will sprout new roots.
Yes, all the way under the cotyledon leaves there will be a kind of protective casing. As it matures, if this is not buried, you will see it poking little white bumps out all over. This is where the roots want to come out. When you bury them, do it up to the cotyledon leaves and they will grow more roots.
I always do this as im height restricted in my grow space. I usually wait until the bottom two or three sets of leaves have died off, or I have enough growth to be able to trim them off, then I bury the stem when I transplant right up to the next set of leaves. Usually means I get an extra 5-6" of growing space.