Let’s pay a tribute to the triangle pose. It is a difficult one, although might seem “easy”.
We usually see the extended pose (utthita trikonasana) in yoga pictures, but before we reach this deep lateral flexion, we want to start with a basic triangle.

Lateral flexing postures stretch the sides of our waist, side abdomen and middle, thoracic, and lower, lumbar spine. They also contract and strengthen the corresponding muscles on the opposite side of the abdominal wall, both anteriorly and posteriorly.
Because they work simultaneously on both sides, they have a balancing and energizing effect.

While one side stretch, the other side contract. While one side is cooling, stretching, spacing, releasing stagnant energy stored in the muscles and organs, the other side is warming, as these organs are compressed and stimulated.
Thanks to that, Triangle pose (and all side stretch poses) work in a way that is balancing and energising, both Yin and Yang correspond equally, balance the body mind and can serve to energizethe nervous system whilst releasing stagnant energy stored.

Gravity also works in this posture, we resist it and stabilize our body against it.  

Try and feel it.

Stand in a ‘leg’ distance apart on the mat.
You can actually measure it, sit on the floor and count the tiles for the length of your leg, then stand between these tiles. Or estimate/measure the length of your leg with a measuring tape.
 
One foot toward the front short edge of mat, the other is perpendicular to the back short edge of mat.

Your hip bones are to the front and your back is straight in the middle (not tilting your pelvis to the sides or front /back).

Bring both arms up to the line of your shoulders. Away from ears.
Look toward the front edge of the mat and start tilting only your upper back to that side.
Gliding to the side until the limit of your reach.
At that point, shift your arms like callipers, one arm down, the other up, opening your chest to the front. 
You are now in Triangle. Breath and feel the resistance from gravity, trying to pull you toward the floor, but your resistance is holding you up and strong.
Your gaze can be toward the upper arm or to the center (keeping the neck in neutral position).

Think about the stretch on one side, try to reach further but to your limits. Think also about the other side, that is being compressed – try to space this side, think about opening this side and not let it compress too much and effect your chest opening. Sometimes this side pulls us forward, stay in the middle line open and strong.

This is Triangle, and it is difficult enough even in the basic pose, before we go into the deep extended one.
Sometimes during a yoga class you will see everybody trying to move into the extended triangle quickly, but remember, it takes time. The more you work basic triangle, your extended pose will be better and deeper, keeping the basic alignment.

Also remember that triangle is unilateral, and we have to repeat the other side as well;
for the same amount of time (or longer/shorter, depend on your spine scoliosis and inclination).

Triangle with leg distance apart, and with smaller distance