I first tasted choi jhaal at a restaurant near the KUET (Khulna University of Engineering & Technology) campus. Their mutton curry with choi jhaal was one of the most popular dishes.
The choi pieces in the gravy looked like cinnamon sticks and they also tasted a bit like cinnamon, but without the sweetness. At the same time they had a distinct spiciness to them which I could not compare with chili or black pepper.
They did have a pungent taste like mustard or wasabi, but not as sharp.
One of the servers told me that the locals like to put in small bits of choi jhaal in a mixture of puffed rice and chanachur and eat it during winter. The chui jhaal pieces add a nice crunch and mild heat to the dish.
Choi jhaal is a creeper which grows on other trees. The stems and roots are edible and the taste of chui jhaal can vary depending on the kind of tree it grows on. It is not used as the main spice; it is used to make the dish more flavourful.
Not only cinnamon, there are other spices like choi jhaal which are not fiery hot, but have a certain heat to them, such as nutmeg, mace, clove, and ginger. Just like choi, they also add a delicate yet distinctive taste to the cooking.
The choi jhaal barks are dark brown, almost black with a sort of fungal growth on them. Apparently the fungal coat keeps them fresh. They smell like musky wood but not very strongly.
A shop owner in Khulna informed me that there are two types of choi jhaal, the more expensive ones are bought by tourists like me and the cheaper versions are bought by local hotels. He also told me that choi jhaal can be put in the freezer and stored for months.