I agree with the answer which explains the subterfuge above but don't think it explains how - you actually get to see the rebel subterfuge earlier: After the rebels capture 'Unidentified Scout Trooper' by tricking him into running around the corner into their waiting arms, they enter the bunker. As they do this, behind Han, Leia etc you can see a person dressed as a Scout Trooper (by Han's leg) standing at the doorway facing outwards, adjusting his helmet as he pulls it on - presumably a rebel waiting at the doorway in case any other Imperials turn up. I have attached the closest picture to this I could find online, where you can see the rebel\trooper waiting behind the main characters, but watching the film you'll see it much more clearly including the adjustment of the helmet.
Anyway, I'm glad I can still notice something new when I watch the films after 31 years.
