the one-way bearing sits on the starter shaft on the engine, inside the backplate on the motor. in normal operation, it locks on the shaft only when it is turning in one direction, which lets you turn the motor over with your RotoStart or PullStart, but allows the starter shaft to freewheel inside. if this gets oil on it, or gets worn out, it will slip on the shaft when it's supposed to lock. sometimes, it will even seize to the shaft, not allowing the motor to turn over on its own, since it is trying to turn the RotoStart.

the only way to get out of using a one-way is to use what is called a starter box. in this case, there is no starter shaft on the back of the motor (where your RotoStart backplate is), there is just a plate that seals the back of the motor. to start the motor, then, you have a box with a motor in it, and a rubber wheel sticking out the top. this wheel sticks up through the hole in the bottom of your chassis under the flywheel, and spins the flywheel to start the engine. This method makes the most sense for racing, since it allows the least amount of resistance on the motor (when running), and therefore the most power. but, this system is more expensive, since you have to buy the starter box and the the backplate, costing AT LEAST 50-60$

So, i suggest you call up your lhs, and see if they have one-way bearings for an hpi T-15. you don't need a new RotoStart, just a new bearing. so, 15-20$ later, you're back playing in the dirt.