In the 11th doctor's 1st series there is a story arc involving cracks appearing across the universe and removing people from history.
However I don't understand, in some examples people are removed from history (the troops from the Byzantium and Rory), however in other places beings appear to be able to enter and exit the cracks at will (Prizoner Zero and the Doctor when checking for debris inside the crack).
Is there any opinion on why the cracks have different effects on different people?