The hard part is determining what is the tip of the spike. I much prefer to advise people or investors to set a target, and cash out at that target regardless. Wait, until Bitcoin takes a hit again, and that's your next investment start point. Currently, because of how volatile Bitcoin is, you get these opportunities. However, this will likely change, and become less dramatic over time, which is when I would advise hoarding it, would be the best option or simply by that time using as a currency might be a little more viable.
This is the problem with buy low sell high, agree. It seems so easy to do, especially now when we're in a cycle and can see the market go up 2 or 3 times from 2017 ATH. But I always feel afraid of selling too far from the tip, and what if the rally just goes on and on and then you end up never being able to buy back in? Then your journey is "over" and you didn't enjoy the top.