Remember that brained enemies will make an unbrained move if the brain tells them to wait.
×The brains aren't on force arrows, so the brains are telling the snake to path through them. The brains will tell the snake to wait if every move that the snake would make would take it further away from the path leading to Beethro (i.e. further away from the closer pair of brains).
The exact position that you want is:
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×Beethro stands at (3,1). Snake's head is at (3,6)
and tail is at (3,7). Step off of the pressure plate.
It shouldn't matter where you step for this to work. This solution also works mirrored on the other side of the room with Beethro standing at (7,1).