This is the promise every "old guard" soldier makes.
It is called the "Sentinel's Creed".
"My dedication to this sacred duty is total and wholehearted.
In the responsibility bestowed on me never will I falter.
And with dignity and perseverance my standard will remain perfection.
Through the years of diligence and praise and the discomfort of the elements,
I will walk my tour in humble reverence to the best of my ability.
It is he who commands the respect I protect.
His bravery that made us so proud.
Surrounded by well meaning crowds by day alone in the thoughtful peace of night,
this soldier will in honored glory rest under my eternal vigilance."
21 steps on that carpet, btw. No more, no less, every single time. (count the steps by the soldier being relieved)
It is a walking man's allusion to a continual 21-gun salute.