actually, with a trainer the line can be flexible or stiff. it doesn't matter. however, once you get past a trainer, you want it to be stiff. here's why.
what happens when you go down a steep dive on a roller coaster? you want to fly upward. so when you are going a dive or pointing downward steep enough to drain all the fuel to the front of the tank, it is already being pushed against the back. i had the same question wehen i had to replace the tank on my trainer. i thought all the other tanks were defective because the line couldn't drop down line they could in the instruction manual.