[Anime Spoilers] A quick question before the season finale ...

Characters on the show have been saying that "there are no cracks in the walls, just like hardened titan skin." But, if there are no cracks in the walls, then how could the soldiers use 3DMG on the walls? If the walls didn't crack, then the sharp hooks would just bounce off of the walls and the soldiers would fall to their deaths. In order for the hooks to hook into the walls, they must have penetrated and cracked the walls...