The Impulse Deflection Crystal:
Like Warp Drive the Impulse Engines are a type of field propulsion system. The Fusion reactor ignites the matter and the energy from that reaction is then colaspsed into spatial disturbance that radiates out from the reactor. Only certain materials react with the impulse wave. Impulse Drive coils can be used to repel or to be attracted to the distortion wave
Residual plasma is flushed to the rear in the Impulse manifold on the exterior of the ship.
The Deflection Crystal cancels out residual wave activity on the opposite side of the drive coils.
The Additional Conduit routed to the impulse drive coils and manifold as seen in the illistration allows the coils to be powered by warp plasma or for the core to flushed incase the core is scrammed mid-reaction
Memory Alpha insist that the Impulse Engine is an ION Chemical Propulsion Thrusted Exhaust system. We've never seen any sort of thrustered exhaust at all or even the need to follow physics and pivot to move in any particular direction. The Enterprise A, Voyager and the Enterprise D were more than capable of reversing engines and direction of travel without reversing the ships orientation on many occasions.
Its my theory that the Impulse wave created by the reactor has to be countered as the ship travels through the disturbance. The deflection Crystal produces and interference pattern on one side of the reactor while the driver coils are on the other side being pushed by the leading edge of the shock wave. As the ship travels through the center of the distortion field to the other side of the shock wave the deflection crystal's interference pattern prevents the wave traveling in the opposite direction from pushing against the driver coils and subsequently allowing for a smooth ride.