The Position Keeper feature was created because the real sailors only had
Manual targeting. Getting the data for the solution (bearing, range, AOB and speed) is time consuming and getting a new set of data can ruin the older set. So they did it once, activated the PK and retracted the periscope as it can be detected by radar. They raised the periscope once a minute to check if the first solution is still ok and updated the components only if needed. If the solution was good after 3 such attempts, the torpedo was good to fire.
The position keeper is reinitialized every time you resend one of the components to the TDC.
On manual targeting, sending the range also sends the bearing (as it says in the tooltip of the Send button).
When you use
Automatic targeting and Position Keeper, you usualy point the periscope to the center of the ship, activate the PK and use the
spread angle to aim the torpedo more at the front or the back of the target. You can see the effect of changing the spread angle in the attack map.
The position keeper is reinitialized every time you turn it off and then back on again.
The spread angle is useful on manual targeting also, since you can change the torpedo's course without altering the solution (and/or the PK) in any way
.