Sanchin Dachi (the Three Battles Stance or the Hour Glass Stance) is a very strong stance that is also very difficult to master.
Begin this stance by placing one foot slightly forward of the other such that the heel of the front foot is parallel to the toes of the back foot. The feet are generally about shoulder-width apart. Now turn both feet inward so that the toes of each foot are turned about 30° inward. This may seem to be a fairly awkward position at first, but it forces your weight down and onto the inside (arch areas) of your feet.
Now the feet are twisted such that great pressure is exerted on the toes, instep, and heels of the feet. This twisting motion is also conveyed up the legs through the knees and to the hips. Your weight should drop reasonably (not excessively) and your posture should be very erect. Your hips and shoulders must remain vertically aligned and straight. Now all of the muscles of your legs, lower torso, and middle torso should be strongly tensed.
This affords a very strong and stable stance that is often used when in close contact with an opponent. The stance offers strength and great resistance to movement. It also allows for greater power to be conveyed into short-range strikes that are delivered from this position (as opposed to strikes from Kiba Dachi or other more open stances).
There are many martial arts styles (e.g. Goju Ryu and Uechi Ryu) that use this stance as a foundational element of their style (much as we use the Sochin Dachi in the same way). It is important to practice this stance frequently as it may take a very long time to master.
It should be noted that the stance can be practiced in the very tense method described above (and some martial art styles make this a focus of development), but this is to develop strength and precision in the stance. In practice, one would only have this tense position for a brief moment while manipulating an opponent or delivering a strike.
You will notice that Sochin Dachi and Sanchin Dachi have very similar spellings, but these are indeed very different stances.