Is there anything fundamentally important about going below 50, as opposed to slightly above 50 values?
I don't believe of course that it's a rule written in stone. However, the correlation seems very strong (see below). My explanation is this: Option prices reflect the amount of fear in the market (because you will probably buy options if you want to either protect yourself from a fall or to be able to buy for a good price but aren't sure if it doesn't go lower). If option prices are low, that means that there's not much panic to be able to be triggered, the market participant are relaxed and aren't staring at the chart all the time for a possible crash. This makes it difficult for bears to penetrate a support, and easy for confidence to build up. So basically the "rule" could be: the lower the lows in the implied volatility, the higher the probability of a substantial price increase.
Volatility is periodic, but just like with the price, there's no clear patterns that could allow predicting the next movements with high confidence.
Let's examine the BitVol and the price evolution in a single image:
(sorry for the weird look of the price chart, had to stretch it because the BitVol image has an "irregular" x-axis).
We see that while there were some instances where a "mid-level" volatility of 70-100 led to a decrease in price, there were only two instances where a value about or lower than 60-70 led to a crash or substantial price decrease: the first in early 2022, i.e. before the Terra/Luna debacle. One could argue that Terra/Luna was an anomaly: there was a lot of confidence before that price could recover, but an "external" event, a large sale of BTC by the Terra/Luna project and big turbulences in the altcoin market led to the crash, and then the late-2022 period before the FTX crash (the sideways market around 2022), which had also low-mid BitVol values, but again we have a strong external event triggering the crash here. All other instances of low BitVol values led to price increases.
Macro-trends like those reacting bullish to halvings (in my opinion caused mostly by price increase expectations and not by the slightly decreased supply, as I'm not a fan of the S2F theory) have of course also influence on confidence, and thus on option prices. Thus, the low volatility is not necessarily "causing" a price increase, but it's a "byproduct" of the increasing confidence due to other factors.