Once they arrive at your set waypoint, torpedos will activate and run faster, thus consuming more fuel. If distance is critical by nearing the maximum range then it's better to place the waypoint enough ahead of the target so the torpedo has only little active running time. This will also avoid premature detection of the torpedo by the enemy sub/vessel.
'Ahead of the target' means: once the torpedo has arrived at the waypoint it should be near the target; so you need to anticipate this by calculation when setting your waypoint, also there is some aid in the game by a little circular point when chosing a weapon to fire, set the waypoint around there for best results, however, this aid assumes that course and speed of the target remains unchanged.
Note that there is a remaining time counter that tells you when the torpedo runs out of fuel and self-destroys. Some AI subs like the Alfa which can run at 42 knots have good chances to outrun your torpedo if you set the waypoints carelessly ... You always need to calculate the relative speed of your torpedo (torpedo speed - target relative speed at the same course) to estimate the ETA at target and see if it's even worth to fire at the target.
Oh, in real subs, multi-million worth equipments will do this job for you
Also once activated and still the wire is intact, other than controlling the torpedo's direction with the cursor keys, you can control the torpedo in the weapons tab in the 3 choices (activation method, turn left or right after jamming, run deep/run shallow).
The various speeds of torpedos when on wire and once activated can be seen in the weapons.txt file.
I hope this helps you, cheers,