This question came up in the LinkedIn ACT Fanatics Forum recently.
You create a new Contact record in Contact Detail View
You know, or suspect, there may already be a Company record set up for the organisation, so you click the button next to the "Company" field in Contact Detail View to display a drop-down list of existing Company records.
You select the one you want.
Result; ACT! copies the Company name to the Contact | Company field and links the Contact and Company Records.
Intuitively, what you would expect to happen is that when ACT creates the link it automatically updates all the other linked fields (i.e.. the address, web address, phone etc) as well at the same time.
In fact, this doesn't occur. You have to now type the address in manually, or click into the Company Detail Screen then Update the Linked fields from there, which is a little crazy.
If we want people to actually use the Company level records we've really got to make the process easier to manipulate