You can select and modify the control poles or polygons of any type of face
(B-surface and non-B-surface) without having to extract, untrim, refit,
replace, or otherwise convert them.
Changes you make to a face are cumulative as you alter its shape.
For example, you can modify a face by first changing its shape along its
U-direction isoparametric curves, and then modify it further by changing its
V-direction isoparametric curves.
Note:The I-Form command creates an I-Form feature in the Part Navigator when
the Associative Freeform Editing preference is enabled in the Modeling
Preferences dialog box on the Freeform tab.
How I form command is works in Nx Siemens mentioned below video?