A guard of honor passes out as Queen Elizabeth II rides past during the Trooping the Colour parade, 1970

 

In the strict world of British military protocol, there are rules for sedating with dignity. There are two main reasons Guards of Honor go outside: It can get too hot and they'll lock their knees.

Usually, it is the combination of both that you get. And really don't "kneel off the knees!" This is the advice given to soldiers standing in formation for long periods in summer.

By locking your knees, you are placing all your weight on the bones and your muscles are not working. The blood flowing to your legs has the benefit of the heart pumping there.

Once there is blood, it needs leg muscle action to help pump it back to the heart. Locking your knees makes it even easier to stand upright, but reduces the use of your leg muscles in standing. This causes blood to pool in your legs, effectively taking it out of circulation.

If your body is moving, the muscles contract and this pushes your blood through the veins towards the heart. If the knees are closed and there is no movement, contractions do not occur and thus the amount of blood reaching the heart is reduced, which also reduces the blood supply to the brain.

When the brain senses a dangerous drop in oxygen, you pass out. An intuitive way of conceptualizing it: The blood flow isn't fighting gravity when everything is at the same level. By going out, your brain itself, feet and heart are all on the same plane, so don't rely too much on leg muscles to overcome gravity.

In the parade, the guard of honor is required to keep his knees slightly bent, and he must move back and forth on his toes and heels, with his feet opposite at all times. By bending the knees and moving on the feet, his blood keeps on circulating.

Basically, the guard of honor should stand on the toes of his left foot while he is on the heel of his right foot, and gently reverse the position. All he has to do is do it subtly, slowly, and very slowly so that it isn't noticed (although parade boots usually have enough room to get away from it easily).

If the Guard of Honor loses consciousness while his knees are locked his body collapses like a straight board that is tilted. When he hits the ground his head will be spinning at a very good clip and he can be injured quite badly indeed. The falling face-first often ends with the terrifying sound of teeth cracking.

This photo was taken during Trooping the Colour, the entire domestic division's annual parade. Part of that parade is an observance, with the queen riding around the soldiers. The picture has been taken on the other side of the same, where she is observing him from behind.