All recent diesel cars have an electronically managed diesel pump. This pump responds from signals processed by the ECU on factors such as engine speed, turbo boost and air flow. The diesel pump injects fuel at a rate in response to the ECU's information. The Rapid TDi kit intercepts signals between the ECU and fuel pump and by optimizing fuelling it substantially increases torque and power by up to 25%. 

Diesel cars are notoriously under-powered at low-mid range. This is as a result of the car manufacturers engine set-up. With a Rapid kit, you should expect a significant improvement in torque throughout the torque band but noticeably at low-mid range.
The optimisation of fuelling can actually lead to a decrease in fuel consumption.

Chips work by re-mapping the ECU. They have their place in the market but the only drawback is that they cannot be removed without going back to the dealer who fitted it. Also they cannot be switched on and off. Thats what makes the Rapid TDi kit better, the fact that you can switch it off. Rapid have created a unique switch for every boost kit which allows the kit to alternate between the modified and original settings. This is ideal when the car needs to be serviced, left unused for a long period of time or you just don't want the kit to be operational. Every kit sold and fitted comes with a 12 month guarantee.