Now I understand--thank you for the explanation. If there's a better word in English, I don't know it, either! The intent of grafting so close to the seed and then burying the graft union is not to grow new roots above the graft (although I suppose that is likely to happen over time, even without etiolation, since avocados can be air layered), but instead, it's simply to protect a less-cold-hardy rootstock from freezing temperatures. I have wondered if there could be any detriment if the scion did end up growing some of its own roots, such as losing salt tolerance or becoming more vulnerable to root rot, but I simply don't know.