@VGThuy , if the free leg is straight, it needs to be at least 45 degrees from the vertical. The angle is between the two legs.
If the free leg is not straight, but bent, the thigh and shin must form a right angle (90 degrees) or more. This isn't as common, but to me it's very similar to a Yoga move I do, where I stand straight, bring up my knee so my thigh is parallel to the ground, and then move it to the side of my body, so that my standing leg and free leg thigh make a right angle when looking in a mirror. My shin would just need to be vertical, or extended out. If it bends inward at all, I wouldn't get the feature.
Basically, the rule is trying to prevent skaters from having an acute angle between the thigh and shin, as this is a much easier position to be in. I can't think of an example off the top of my head, but many teams take this option for the Group B feature.
Edit: Here's a poorly drawn example.
drive.google.com