The red button "PK" is Position Keeper. Purpose of that is to keep the TDC plotting the current position of the target when the periscope is down. In real life they raised the periscope for 10 seconds or less to take range, bearing, and estimate of angle on the bow, then lowered the periscope to make it less likely some alert lookout would spot it. If you leave it up for 10 minutes, sooner or later someone will notice it. 10 seconds every 2 to 5 minutes reduces the chances of anyone seeing it.
So the Position Keeper tracks the target - but assumes that the target is not turning, speeding up, or slowing down, but maintaining a steady course. That's why it's necessary to note the position, bearing, range and speed on the TDC, then click the red button to turn it off, raise the periscope to take all the readings again to confirm the accuracy of the plot. And turn it OFF before firing, the "FINAL BEARING AND SHOOT" observation has to be real time rather than the prediction made by the PK, although they should match pretty close.
Again this bears repeating because it's important, hit the Q key before shooting to open the outer doors - if you let the game automatically open the outer doors, the torpedo will be aimed at where the target WAS, not where it IS, when the torpedo leaves the tube. Single biggest cause of misses behind in this game is failing to check outer doors open before firing.
For manual targeting, main reasons I don't use it is because (1) I find it very difficult to estimate angle on the bow within 20 degrees, and (2) the AOB changes very rapidly at closer ranges. With manual targeting the range, bearing, AOB, and target speed have to all be correct when you fire, if you have the AOB for the previous target still set or the AOB is correct but the target sped up, slowed down, started turning, or danced the Macarena since the last inputs, the gyro angle on the torpedo will be aimed the wrong direction.
Essentially that means you have to turn off the PK (the only thing it's good for is keeping track of a single target when you're not looking at that target, helpful when plotting but not when firing) and check all the numbers for each target. And hit Q then W to cycle through each tube and check the message "OPENING TUBE (number)" for each tube before starting the actual attack.
Keep in mind if it was easy, everyone would be doing it.
