It definitely can, but it does not always occur. Depending on how your knees are shaped, standing with your knees bent for just 10 minutes can sometimes make you faint. If your knees are shaped a certain way, then locking your knees pinches a major artery just slightly, stopping a small amount of blood to stay in your legs instead of going to your brain. The longer you stand like that, the less oxygen goes to you brain. First you will feel woozy, then you might feel very nauseous, and then you wake up on the ground, wondering what happened. Just recently during a choir performance I, not knowing any of this yet, stood with my knees locked the entire time, because it uses less muscle. I felt like I was going to throw up, saw colors, and then I woke up on the floor with three other choir members trying to help me up.