That would depend on how the helm achieves directional control of the ship. If it regulates the balance between port and starboard warp nacelles, then a diagram like shown here would be a natural part of the operations, at the fundamental level that Spock and Chekov are diagnosing.
What other means could there be? The ship might have a futuristic rudder of some sort, but reference thereto in ST6:TUC appears colloquial at best.
Well, let's address that scene.
SPOCK: There is one possibility. The automatic helm navigation circuit relays might be disrupted from engineering level three.
SCOTT: Aye. I can take them out and cut in the manual override from there.
They think they disrupted it from inside a tube but it didn't work. Spock wants Chekov to verify their work. Chekov looks at the diagram in question.
CHEKOV: All indicators are dark.
SPOCK: Thank you, Ensign.
SPOCK: It appears, Captain, we've been doing what used to be called pursuing a wild goose. M-5 has rerouted helm and navigational controls, bypassing this primary system.
SCOTT: But it was active. I'd stake all that I know that it was.
SPOCK: I believe that when M-5 discovered our tampering, it rerouted the controls, leaving this one active by simply sending through an electronic impulse at regular intervals.
In other words, Chekov had no reason to look at a "warp engines and reactors" diagram. He had reason to look at the helm and navigational control diagram which would contain the "automatic helm navigation circuit relay" and probably the "manual override" circuit.
As to how
the ship is steered and what
powers the ship on a granular level, that is beyond the scope of the scene and diagram, IMO.