The B1 Recce is a conventional cocoon-style bivi bag, with a zip running around the head area and down the left-hand side (the same as on most sleeping bags).
The key to the performance of the B1 Recce - keeping you and your sleeping bag dry - lies in its X-tex fabric. X-tex fabric features a polyurethane (PU) coating impregnated with activated carbon. This has an innate ability to absorb moisture, stay dry to the touch, and continue breathing, expelling humidity to the outside. You can read more about X-tex fabric here.