Cedar is regarded as a soft wood, so because of the resins it is not recommended for smoking. Planking does not release the resins into the smoke.
Cedar is a soft wood. similar to pine. Burned as a cooking smoking wood, it can ruin your pit by depositing tars and a cresote on the pit walls.
A soaked cedar plank under a salmon doesnt ignite, yes it chars on the underside a bit, and the flavors from the aromatics in the wood comes through the cedar and penetrates the salmon, but the wood never actually ignites and burns. The small amount of charring you get is enough to release the oils through the wood but does not produce any significant amount of smoke. You will notice that the entire top side of the plank stays intact as well as throughout, and only the bottom chars up a bit.