Force Sword’s “Warp Resistance” is not effective against “Kinetic Deflection”.
Use of this weapon increases a Psyker’s Peril. The higher the Warp Resistance, the less Peril is generated with each attack.
this has been reported before and i believe it is intentional, the warp resistance just effects the warp gained from special activation. as it stands it is kind of a useless stat that you may or may not care about depending on build.