If I remember right:
biz.BasicForm.Data will return the data that is currently in the row.
biz.BasicForm.FieldEditingControls["FirstName"].Value
will get the value of a textbox or control prior to insert or update.
So essentially if you want to do validation prior to insert, you'd use the
.FieldEditingControls["fieldName"] and then perform the
.SaveData() method, then you can use .Data to retrieve the data submitted.
I could be wrong, its been a while since I've worked with biz form API.