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All REL's and most other decent subs have this switch. If you engage the crossover bypass it will switch off the sub's internal crossover filtering and the sub will rely on the signal being filtered by the amp instead.
If connecting up the sub to the amp by the LFE cable only it is preferable to switch out the sub's crossover and rely purely on the amp as there can sometimes be problems caused with cascading crossovers if both crossovers are engaged.
For those that don't have the bypass switch you can achieve the same effect by switching the sub's filter as high as possible.