Yea, also you must be very carefull not the lock the wheel up when you downshift or that your spring setup is correct becaus if the wheel stops moving then the force of the rear caliper is no longer tranferred to the swingarm but it then works as a normal caliper.
I wonder how Jan Greven managed to ride this in a race
Well it actually force the swingarm towards the ground when you use the front brake.
That instead of the backend of the bike that raises (like in a normale situation) you will also force the swingarm downwards.
Application of the rear brake has no effect on this.
Having free sag in the rear shock does. Once the swing arm has fully extended under hard front breaking, there is nothing you can do to force it towards the ground other than letting off the front brake a little or trying to roll your hips backwards to transfer for weight to the rear of the bike.