You do your best to keep them in a flat line. You keep them from moving in a circular motion with the knees bent. A lot of hips movements can be reduced with a little effort.
There are several ways to hold your knees together. Your knees can be held in front of your torso, behind your shoulders, or above. There are various ways that knees may be connected to hips.
A friend of mine would often point to his wife’s knees. He’d ask her to point and she would do so with her knee bent at about a 90 degree angle. It’s a very nice sight because a woman’s knees are very close to the hips.
However, if a man were to point to his wife’s knees, I suspect her mind would automatically tell him to point to her crotch and her knees are closer to her ass. I think this would be very hard to do.
Keeping knees together is easy when you do it right. Your knees can be held together by pressing them into the ground. The hips can be supported by keeping your pelvis in the neutral position with your lower pelvis and lower abdomen close together. If the pelvis is a little higher than your lower lower back, keeping your pelvis at about the same distance from your lower lower back helps to keep your hips from moving forward. However, if the pelvis is not as close to the spine as you can keep it, the hips can move farther forward too. If you keep your hips and pelvis as close to the spine as you can, you can work on the pelvic tilt by raising your hips. Keep it simple by keeping your pelvis closer to the spine. You don’t want to get so high that it makes it difficult to point.
Keeping knees together and maintaining a vertical back should not be too difficult if you’re doing things properly. You want to keep your knees close together but you also want to work on the pelvic tilt because of the pelvic tilt, not vice versa. It’s easy to put on a show because you’re not having to really think about the pelvic tilt. Instead your mind is free.
The problem comes when a man moves to a different angle from a woman. If you do a split push-ups or a chin-up to reach a certain angle, you’re putting the knees in a way that’s difficult to keep them upright. If you were to hold your knees together then slowly reverse direction of the movement and then do the opposite,