Locking your knees causes more blood to pool in the lower extremities so less of it reaches the brain. It normally takes at least several continuous minutes of standing with your knees locked like this to faint. It sometimes happens when singing in a choir, or standing in formation, because

a) you have to stand in the same position for extended periods of time,

b) your body is consuming oxygen faster because of singing, a warm uniform, and/or hot weather, and

c) you may be so focused on your task that you disregard your discomfort and don't even realize you're locking your knees or feeling faint.

When I fainted in choir rehearsal, I think I was only unconscious for a couple seconds. I fell forward on my hands and knees and came to my senses at the choir director's feet. I felt really flushed and clammy at the same time, almost feverish, and it took a few more seconds after I regained consciousness for my vision to clear. Basically as soon as I was horizontal and normal blood flow resumed, I started to recover immediately. But if heat stroke or other factors are involved, that might not be the case. I don't envy that soldier being carried on a stretcher in that picture; I don't know what kind of environmental factors or underlying conditions may have been involved.