A swing will rotate it sideways. Imagine the plane extended until it passes by the camera. If it is far from parallel to the groundglass, it will pass relatively close to the camera and a considerable swing or tilt is needed. If the plane is nearly parallel to the groundglass, it will pass far from the camera, and a small adjustment is required. When a tilt is used, it is common for the plane to pass the camera many feet underground and the appropriate adjustment is very slight.
Novices often start with too much.
The initial estimate for a swing or tilt is likely to be close to correct if you visualize the plane of the lensboard and the plane of the groundglass out of parallel and extended until they intersect. The goal is to have them intersect in the plane you want sharp. If you succeed, you will have satisfied the Scheimpflug principle and the swing or tilt will be correct. Usually, though, some further refining is needed. Section 8. will provide an efficient way to do that.