Since the bag expands to the size of the tank you can feel the fuel stop going into the tank. If you use a pump you run the risk of bursting the bag and damaging the tank.
There seems to be a misunderstanding of bladder tanks. Pylon racers use the bladder tank with just two lines. Fill/Feed and pressure. This configuration requires a syringe to completely empty the bladder and then fill it. The Jett tanks are two line and meant exclusively for Pylon Racing.
The Tettra tanks can be set up in a two or three line configuration. The Tettra instructions show only the three line configuration. Fill/Feed, Pressure and overflow. They fill, feed and empty just the same as any other three line tank. Whether the pump used to fill is hand or electric there is no more danger of bursting one of these than there is of any other tank. Fuel will pour out of the overflow when the tank is full. The overflow has to be plugged when the tank is full. Dare I say it again? Just like any other three line tank!
Ed S
The picture shows a selection of Jett and Tettra tanks.