Thrust angles are used to offset things such as engine torque, and the tendency of a plane to climb excessively under power (especially trainers). So, trainers usually have a bit of down thrust built in, and right thrust built in (down to couteract the high lift of the wing, and right to counteract the torque of the engine, which tries to pull the nose of the plane to the left). Thrust angles can be either built into the firewall when built, or can be added by adding washers between the mount and firewall, either on the top two bolts (for down thrust) or the left to bolts (for right thrust).