Musubi Dachi is very much like Heisoku Dachi except that the toes point outward at about 30°. The heels are together and the hands and knees are held in the same manner as Heisoku Dachi. This is commonly called the Knot Stance. In some martial arts systems this is also used as their Ready Stance.
This is a somewhat more stable stance than Heisoku Dachi, particularly when you have been contacted directly from the side. But it would be invalid to assume this is a stable stance. It is only slightly more stable than Heisoku Dachi.
Like Heisoku Dachi the Musubi Dachi is primarily used as a starting or ending point for some activity. Commonly at the start of classes, or the beginning of a Kata.
Musubi Dachi is unlikely to be used in a conflict situation as it has been defined above. While you may move into or out of Musubi Dachi as part of a movement sequence it would be impractical to have your hands down. Like all stances, the Musubi Dachi is held only for a momentary purpose and is then abandoned in favor of the next action or activity.
In some martial arts systems the Musubi Dachi is incorporated as part of an opening sequence for Kata and other activities. The practitioner begins in Heisoku Dachi, then by moving only the feet or ball of the foot transitions sequentially to Musubi Dachi, Heiko Dachi, and Hachiji Dachi. We do not have this practice in Tensoku Ryu but you can readily notice it when you watch videos of practitioners from various styles performing Kata. This sequence is simply part of that martial art’s culture.