I wouldn't say freezing the bark is important, it's just an extra step you can take to increase yield even by a small amount.
Rock can be cracked and split with frozen water, it can be cracked with heating. Long ago when tooling was primitive rock was split this way using fire, nature does it with ice.
The key factor is expansion which can be achieved with both high temps and low temps. In freezing plant matter the expansion comes from freezing water, which as mentioned already, helps break up the cells more than just boiling. In a sense you get more cell destruction from a greater range of temperature application.
I haven't done a freeze myself but I might be inclined to do it in stages. firstly with just water, as impure water freezes at lower temperatures, then maybe do subsequent freezes with vinegar added. It might be slightly more difficult to freeze with vinegar in... I'm not sure. Perhaps some room for experimentation, but either way I'd start with clean water, enough to completely soak the bark plus half an inch above powder line, agitate occasionally once the water starts to thicken/freeze/crystalise to maintain an even suspension.
It might be interesting to try a series of boils and freezes.
Eckhart saw Hell too. He said: "The only thing that burns in Hell is the part of you that won't let go of life, your memories, your attachments. They burn them all away. But they're not punishing you", he said. "They're freeing your soul. So, if you're frightened of dying and... you're holding on, you'll see devils tearing your life away. But if you've made your peace, then the devils are really angels, freeing you from the earth."