Love this pose...seems simple, but it is not. Lots going on and can be a simple prep pose or super duper strong, depending upon the variation. Here are some variations:
That fingertips to the floor thing? Close to impossible. That knee going forward of the ankle thing? Very advanced...proceed with extreme caution!
I recommend many anjaneyasanas if you have any issues with hip flexor/psoas tightness...it works wonders.