Tri equals three, so there is a threefold nature to any of the Triangle positions, whether upwards or downwards. It's Stability-Activity-Lightness: Stability in legs and arms, activity in the middle (our sacrum and solar plexus), and lightness in our shoulders and, most importantly, our head. Our thinking becomes 'light', less bound to the body and will.
Raise your leg behind you and resist the temptation to open up the hips. Keep them square and facing the ground. Raise your leg until you've reached a natural resistance. At this point, don't 'push' your leg through it. Observe your breath until it softens and then guide your leg higher, or maybe not. With free-breath, your body will let you know the next step.
Then bend the raised leg and bring it under your body. If you feel less stable or lose activity in the midsection or get tight with your thinking or shoulders, then you've gone too far. Straighten the leg behind you again and repeat. It really doesn't matter how far under your knee comes or how high your leg rises as long as you maintain this threefold nature. It's a helpful preparation for sun salutations, for eventually swinging that foot all the way down on the mat with ease.