How do I need to cut the taproot?
What happens on top of the plant above the soil – when you prune it you get re-sprouting of the dormant buds – does also happen below the soil: the pruned root re-sprouts, and these re-sprouts do not have the problem of the “stored memory that tells him to grow rounded”. Instead of that, it sprouts and grows again as it does originally = vertically downwards. The new re-sprouts do not have the breaking power that the original primary root has, but they are far better than a superficial rounded growing tap root, that stays in the top soil where there’s none or just a little bit of water available for the plant. This is also the reason why so many trees fall down in cities with strong winds. They are just not anchored.
You can learn everything about how to cut the taproot on this page.