I don't think overtax is a standard term with any specific definition related to motors. It has a general meaning as follows:
o·ver·tax
tr.v. o·ver·taxed, o·ver·tax·ing, o·ver·tax·es
1. To subject to an excessive burden or strain.
2. To tax in excess of what is considered appropriate or just.
If we apply that to motors in general, we might be referring to any kind of excessive abuse of the motor. Probably overload (too much mechanical load). Possibly frequent starts.
Now you mentioned " in case of load decoupling". There is generally nothing burdensome to the motor about mechanically disassembling the coupling to run the motor without turning the associated load.
Unless perhaps it is a vertical motor with upper thrust bearing that requires minimimum downthrust which is not developed by weight of the motor in absence of the pump.
Just a guess. Without more context I don't think my guess is worthwhile. I think you should ask the person who used the term what they meant by it.
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