Flat scissors is not a good defensive manuever for a spit.
I respectfully disagree with this statement. First any good turning plane such as spits can make excellant use of the flat scissor. Second, you cant just blindly fly scissors without some kind of plan. If you do it in hopes of running the guy out of ammo or fuel you gonna get killed.
A proplerly performed flat scissor should take less than 4 turns...(2 each way) before it either works or not. If you haven't gained any advantage by this time then you should have performed another move.
The timing of each turn is dependant upon what the plane behind you does. One must be watching for the angle off tail. When that suddenly increases its time to go offensive. If the target yo-yo's because its working you have options to duck under, exit for speed or offer a bait.
The flat scissor will not work if you are all out of speed and options. The proper time to do it is when you have adequate speed.
It is true you may or may not offer a gun solution. BUT if the other plane goes for guns you will immediatly get an angle off tail. If he misses that is a signal to grab those angles and either exit for speed or go to rolling scissors for your shot.
In any case a good turning plane is the best plane to use flat scissors. Further, flat scissors can be and often is an "offensive" move because you are controlling the fight at that point. What you make of your control depends on whether you have a plan or not.
no plan = dead
plan = options