Drag a shim, or click it and use the arrow keys. The curve beside the tube is the field across the sample — that is the thing you are flattening.
A shot redraws the lineshape and costs instrument time. The lock trace is free and follows the shims as you turn them — which is how you actually shim.
| width at half height | — | |
| width at 0.55% | — | |
| width at 0.11% | — | |
| lock level | — |
These fail independently. A Z2 error barely moves the half-height width and still ruins the base — which is why the hump test exists at all.