Well, first there are strands of magnetic field left inside, but now the superconductor doesn't like them moving around, because their movements dissipate energy, which breaks the superconductivity state.
首先呢，有几束磁场束会留在超导体里面， 但现在超导体不喜欢让它们在周围移动， 因为它们的运动会消耗能量， 这样会破坏超导体状态。