How many forms of trikonasana are there? It depends on what style you practice and if you use modifications or props. There are many modifications of the pose and you can utilize props if you like. Here is my analysis:
This is the Bikram form of Trikonasana:
WRONG says Iyengar! This form to me incorporates the upper body of an Iyengar/Ashtanga style with Virabhadrasana. I see this incorporation of poses in other Bikram poses, such as janu sirsasana, where he incorporate upavista konasana to stretch the adductors. This could also be a form of Utthita Parsvakonasana in the torso. The Bikram form is more ascending in the torso than the classic form of trikonasana. The arms are still equally outstretched and the bottom hand is reaching towards the toes without touching the floor.
The Bikram dialogue does not give much verbiage into the workings of the legs, just the angle of the feet and the bend in the front knee.
The Iyengar Trikonasana is a very wide stance, as wide as the Bikram without the bent front knee. The back foot is angled in 15 degrees from facing forward. The bottom hand is placed on the floor outside the front shin with the palm flat. There is a deep external rotation in the femur bones set up from the entry position to allow the hip joints to open.
I love the feeling of this pose and the very good approach to open eventually for vasisthasana, which is the beginning of Ashtanga third series or visvamitrasana:
The Ashtanga version is a closer stance, with the back foot angled in a little more to 45 or 60 degrees.
The reach is to the big toe of the bottom foot with thumb, index, and middle finger. The arms, as in the Iyengar version, with spine parallel. I have heard different methods of spiraling the femur bones. Gregor Maehle says rotation both in the same direction to enter and then change the spiraling once the pose is entered. This pose is also a prep for the more advanced asana mentioned above.
Of course, Iyengar and Jois had the same teacher, Krishnamacharya, but have their own approach.
All versions are great poses for strengthening the legs, opening the hips joints, and to continue to develop uddiyana bandha strength. Modifications and the use of a block for the lower arm is great for beginners, but go for the depth and don’t add on unnecessary flourishes, like arms overhead. There is enough work in these poses to open you up for advanced asana variations.