Locked knees causes you to pass out. If I understand it correctly, while standing around casually, you still move around a bit, which causes muscles in your legs to contract and relax, which squeezes your blood vessels and helps pump blood back up to your torso. If you lock your knees and stand perfectly still at attention, your leg muscles are no longer doing that, so blood pools in your legs and you grey out at 1G.

From my own experience in marching band, this is worse in hot weather. I think it's because in hot weather you're more likely to be dehydrated, so less water in your blood = less total volume of blood, and your capillaries dilate in an attempt to circulate blood closer to the skin = more volume in your circulatory system, == less blood pressure in the first place. I never once remember someone passing out during the winter. Happened a lot in the summer though.

Especially on 9/11/2002. Our genius principal decided we were going to have a memorial service on the one year anniversary of the attacks, with all students and faculty in attendance. There was one facility on campus that could seat all 1200 students and 120 faculty at once: The football stadium. And even then it was a tight squeeze to get everyone into the Home side bleachers...no worries though, we'll put the 40 student JROTC and 120 student marching band in uniform on the field itself. They'll play the national anthem at the beginning and then stand there at attention for the entire 45 minute pointless speech. In the direct sun on a hot and humid Carolina morning. 5 JROTC and 4 from the band passed out.

From passing out. Locking knees can reduce blood flow

The trick is tense your leg muscles when standing at attention for long periods of time. Or subtly shift weight from one foot to the next.

It's why some old movies look good even at 4K

Couple other good examples of this: the original Tremors and the music video for "Last Christmas" by Wham. Both look shockingly good for their age.

That's why the Star Trek TNG remaster looks so good. Yay film!

Lol, so disgusted by his lack of decorum and commitment.

I love getting rid of my own braincells! Lack of then is essential in a functional military!

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Yeah it is but high school sports ain't one of them, plenty of issues there otherwise

It's worth noting that their job has zero room for errors. They are expected to be basically invisible, outside of the ceremonial parts. They are also (I believe) authorised for live fire, at their own discretion.

They walk a political tightrope, and the last major fuck up I heard about was decades back now.

Kings Guard is just an assignment given to several regiments of the British Army. They would be doing other duties, training and deployments otherwise.