Volvo and other vendors make a tool (SVO) called a counter hold that wedges into the RWD Volvo redblock low-friction engine with the harmonic balancer crank pulley.
Using this negates nearly all possible risk of damaging the key-way or alignment tooth of thing like the crack shaft timing belt gear and the guards either side.
You'll find many will use a rope trick or, with manual transmission, engage a gear and apply the parking brake. These methods can work, yet, again, you risk damaging the alignment methods of the components hanging off the front of the crank shaft.
The counter hold is Volvo Cars AB OEM part number Volvo Crankshaft counterhold tool #9995284.

Image courtesy of: http://www.etoolcart.com/.
You find Volvo parts from the Volvo stealership, Volvo Penta, SPX Tools, OTC.
http://www.volvotools.com/
OTC may have been called SPX tools. Unsure.
http://www.otctools.com/
There is another counter hold tool used to remove both timing gear sprockets.
These sprockets use a rolled sheet steel as an alignment pin.
Also, using an impact wrench, without a counter hold tool, could also cause such damage. Though many articles report success using an impact wrench on the center nut.
You may check iPd and FCP Groton for aftermarket Volvo tools. Some report poorly made tools that fail to work. While others find the aftermarket tools useful and durable.
Finally, many can make a counter hold tool.
The problem with the counterhold tool #9995284 is that the new Volvo OEM harmonic balancers use "holes" that are smaller than the counter hold tool "teeth" I found. Though I was able to secure without modifying the tool or the balancer.
Questions?
Hope that helps.
cheers,
dud.
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