This may backfire, but let me jump in and give an explanation as someone who has never used a spade bit and never will...not too sure I'll ever graduate to a leveraged bit, and no way I'll ever be ready for a spade. But...
See the bit below:
The spade part is not gouging into the roof of the mouth. It is resting between the tongue and pallet. The reins are attached at the end of the levers, which amplify any movement of the reins.
Given a bit like that, it is reasonable that the horse can feel and respond to vibrations on his tongue instead of pressure on the bars? If, of course, he had been gradually introduced and taught to listen to vibrations on his tongue.
To me, that sounds reasonable. There is a lot of stuff resting on the tongue, and it makes sense that it would be very easy for the horse to feel it.
The mistake is to assume, as I used to, that the idea was that you could use all that hardware to punish the horse for disobedience. If you are thinking that way, then spade bits are not for you - you generic, not any individual on this thread.
If a bit is for communication, then all that stuff might be able to act as an amplifier in the mouth - but an amplifier based on having a large area of contact and with lots of things that vibrate rather than amplifying by putting extra pressure on the bars.
If I'm wrong, I apologize to all. That is how one person who has never held a spade bit is trying to understand its use. Feel free to correct me so I can learn.
If I'm right, it is arguably a gentler bit than what I'm currently using with Mia (Internet pic, this is NOT Mia nor how I use it either):
Edit to add Mia...I'm currently using this on the little ring below the big ring. She seems relaxed so far: