To free up the processor, after a bounce with the effect inserted, you must un-insert that effect to make other effects available for use. You can use a maximum of two inserted effects on every track. The maximum is reduced to 1 effect when you are using MTK effcets, witch are the Mastering TracKs effects.
When you decide to use another effect you have to un-insert it from where you inserted it. Another way to listen to effects, is by sending them as a Post- or Pre effect to the track you like. "Pre" will let you hear only the eaffected signal. "Post" will let you hear a mix of the clean track + the PRE effect in the MIXER mode.
This way the effect will not be burned to your track. You can change it any time you want, because the track is recorded dry.
Hope this is an answer to what you asked.