When two lenses having equal but opposite power are next to each other, their combined power is zero or plano. They neutralize each other. The curvature of the plus lens where ray #1 enters it is exactly the same as the curvature of the minus lens where ray #1 exited it.
 
 

When the plus lens is moved away from the minus lens, the curvature of the plus lens is much greater where ray #1 enters it than the curvature of the minus lens where ray #1 exited it, and so there is more refraction occurring in the plus lens than there was in the minus lens. So, the plus lens has effectively gained plus power.
 
 

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