Complete the section inside the Silence ship in the eighth level, Somewhere in Elizabethan London, without losing sight of the Silence and having to return to the beginning of the section in order to unlock this. Go slowly, and don’t try to rush this section, and you should be fine. If you don't think you're going to make a jump without losing a Silent, then wait until you can. There's no time limit, so take your time and you should be able to unlock this in one go.
You can set the puzzle difficulty to Easy to make the stasis field generator puzzles easier, but this won’t actually affect the movement of the Silence themselves.