Good morning Kevin .
To run the gasifier in reverse for a couple of minutes ( with hopper lid off ) will allow oxygen to the area of the grate to help burn away any fine particles that might be causing restriction . The heat from this upward travailing gas and heat will help dry and start the charring process of the wood above the grate .
There would be no need to operate the reverse blowers and the pusher blowers at the same time . With the hopper lid open , operating in reverse and if the pusher blowers are turned on air is pushed in at the nozzles combining with the reverse flow of the gasifier and the air and gas will exit the lid . Keeping in mind the fire and char will advance toward the incoming air there is a possibility char could form at the nozzle entrance and leaving raw or less chard wood below the nozzle . If the lid is put back on and the gasifier pulled in the correct down draft mode there may be char at the nozzle level making gas that is going through fuel that has not been reduced to char yet . This wood can't burn because the upper layers of char has robbed the oxygen, but heat is releasing gasses and tars from the wood below the char.
To try to make the above concept a little more clear and what NOT TO DO . If one started with an empty gasifier , and added some raw wood and covered this raw wood with a depth of good char . If the char is lite up and used in gasifier mode to run a motor I would guarantee dirty gas and locking up a motor .
In the above NOT TO DO example the upper layers of char are creating gas ( possible enough to run the motor ) and robbing the oxygen so lower levels can't burn, but the heat from above will release all the tars and nasty stuff .
In short running the gasifier in reverse just a little at start up is insuring the proper stratified layers are in the correct order .