The selected frame of the animation is loaded into an OGF editor, where you will be able to edit the frame in detail, including the images themselves. PR's OAF Editor will wait for the OGF editor to finish, and you wont be able to access the Main Window until the editor is finished.
After the editor has finished, a confirmation dialog is displayed confirming whether you want to replace the current frames data with the updated OGF file you edited in the OGF editor. Click no if you don't want to save the changes you made in the editor. This confirmation can be turned on and off in the Preferences dialog.