Update data in a form based on record selection in another subform

I need for data in Form2 to change based on the selection of records in 
Subform1 of Form1.

Subform1 displays continuous records, and when tabbing to or selecting
those 
records I'd like for Form2 to automatically display related data.  Both 
Subform1 and Form2 have a common primary key.

Hope this is clear.  I'm sure I've made this work in the past, and I hoped

existing posts and Access Help would get me back on track -- but so far,
no 
luck.

Thanks.