It'll go all the way to eleven if you don't stop it. Depending on how the engine is controlled which these days is either a DEEC or a FADEC system that allows a simpler operation from a pilots standpoint. They keep the engines from overspeeding, overtemping, flaming out, compressor stalls, synced with each other and a laundry list of other functions. When reverse is selected either by lifting some paddles on the thrust levers or selecting reverse by unlocking the levers over a mechanical stop depending on the aircraft the computers use several systems to determine the next course of action. Usually weight on wheels is required for the TRs to deploy however that's not the case for all airplanes some of which don't others need to have wheel spinup criteria met before allowing them to deploy. Once whatever criteria are met to allow the TRs themselves to deploy the clamshell doors (ex. DC9, MD80) or the cascade (ex B737-300 and newer, A320, B777) will open. Once deployed the engine spins up and the exhaust or bypass air is redirected a direction other than backward. Some is directed forward, some sideways and still some isn't redirected depending on the system. Once deployed the engine controllers or computers limit the thrust produced to whatever the limit is for the engine. Other airplanes require manual actuation not to exceed a certain power setting based on rotation speed of the turbines or an internal pressure (EPR).
The engines themselves don't swap directions of rotations but rather redirect the airflow.
