TDC stands for Torpedo Data Computer. In stock SH5 it's hidden from you; instead of using it personally, you tell the XO what information to add into it. You can see that interface in the photo below:
TDW's UI's reveal the TDC dials so that you can enter the target information personally, as in this photo:
To turn it off, you either tell the XO to turn it off, or click the small red-ringed button in TDW's TDC (actually, the red ring means it's already off; it will be green if the TDC is currently turned off, and will turn red when you switch it off.)