When you construct swept features with a non-planar path and one or more cross sections you can define a locking axis for the cross section profile during the Axis Step. A locking axis allows you to control twist in a swept feature.

When you select a locking axis (A), the cross section profile and resultant surfaces maintain a fixed relationship with the plane that is normal to the locking axis direction, which is constant (B). With no locking axis specified, the cross section profile and resultant surfaces maintain a fixed relationship with the plane normal to the path, which varies (C). In this example, you could also use the vertical line (D) in the cross section profile to define the locking axis.