The Juji Dachi (X-Stance, Cross Stance, and sometimes the Twist Stance), involves positioning the back foot near but behind and to the opposite side of the front foot. Consider standing in a Sochin Dachi and then sliding the back leg forward so that it slides in behind and then to the outside of the front foot. As you do this your back shin will naturally come in contact with the back of your front calf. At this point the front calf and back shin should press into one another to form a stable base. When executed properly there is no space between the shin and calf.
Juji Dachi derives its name from the Kanji for the number ten, or Ju in Japanese. Here is the Kanji for Ju. You may also wish to note that “Ji” means” Kanji character”. So Juji would mean “like the character Ju.” You can also find Ji used in Hachiji Dachi.
The stance is used to glance in another direction before turning, for moving in beside an opponent, and for offering a means of closing distance on an opponent. To close distance one merely establishes a Juji Dachi as described above, and then shifts weight momentarily onto the back foot and steps forward with the front foot.
Juji Dachi can also be established by stepping back with the front leg so that the front foot goes to the outside of the back leg until the calf and shin again make contact. This is an effective way of moving backward.
A similar stance can be created by stepping backward, but moving the front leg to the inside of the back leg until contact between shin and calf. This is not quite the same as it makes it difficult to then step further back from this final position. It does, however, offer the benefit of allowing you to quickly twist and move in a completely different direction, something the first form of the stance does not provide (at least as gracefully).
A similar movement is often employed during sparring or conflict in which the entire purpose of the movement is to close distance on an opponent. In this case the movement does not bother with making contact between the shin and calf but instead rolls directly forward again with the front leg. This is defined as a Cross Step and is a stepping motion rather than a stance (although one could argue that one or more stances were involved during the execution of the movement).
Experiment with different ways of entering the Juji Dachi (for example, just twisting while in Sochin Dachi or Heiko Dachi can invoke this stance). Also experiment with various foot positions, a variety of different stance elevations, and possible transitions to other stances.