Why do my edits to a shapefile disappear when I try to check them in?

Here is my routine:
1. I Connect to the folder with shapefile and add the data to the map.
2. I Select a feature and choose Check Out Feature. Grips appear on object, but I want to use AutoCAD tools to edit it.
3. I choose Extract Geometry from Feature. I get an mPolygon.
4. I Explode the mPolygon to a Polyline and edit it using AutoCAD tools, i.e. Trim.
5. I turn the new polyline back into an mPolygon
6. I choose Update Feature from Geometry. I select the new mPolygon and the feature changes to match my edit. Great.
7. I choose Check-in Feature and my edit disappears. Not Great.
What's the deal?