Howdy,
While there are not any sources I know of that tell how heaters were used, there might be some useful info gleaned from the "Insight from Historical Artwork" section of this page.
http://www.thearma.org/arttalk/ArtTalk.htm
Of course you can only trust an artists rendition of things so much if not specifically under the direction of a fight master, but I went through many of those pictures. I noticed that while most bucklers are depicted at arm's length (or at least in some dynamic position), the heaters were all pulled close to the body at chin level. Allowing for the previous caveat, it leads me to believe that these shields were probably kept in close and the sword was used to displace attacks just like a buckler. Of course a fighter will move his shield as needs be, but that's the impression I get just from these pictures.
All of my heater or equivalent training experience is SCA related, not using a historically accurate shield (or if it was, it was on accident), so it will most likely result in you getting your knees and below hit often. But I can give a couple pieces of advice...
1) It is easy to blind yourself with your shield if you use it cover your head.
2) If you try to "block" with it to far away from your body, it is easy to throw it out too far and leave yourself very open.
3) They sure get heavy! But so are bucklers after a while...
Edit: I thought I'd seen something that did show heater shields being moved about, and it in the pell history article.
http://www.thearma.org/essays/pell/pellhistory.htm
I suppose one could also account for the static look in the above art section as in part being due to archers in the field.
