Canoeing is an ancient form of transport with different forms all around the world, each with they’re own strokes and techniques. And so the are several ways to keep a canoe tracking straight, but most work on the principle of power then correction, whether its foward or backwards. First, supply the power and the canoe turns away from the paddle, then add correction and the canoe turns back to point in the chosen direction. And by controlling the level of correction a canoer can maintain a straight line or maneuver a gentle slalom with it much input from other strokes.
To go straight forward I was taught to pick a object to aim for, then you notice when bow goes of course and then corrections can be made to bring the bow back round. As you progress this correction becomes more and more natural til you no longer have to pick objects to maintain the correction line.
Backwards is similar but slightly hard, as you don’t have eyes in the back of your head. So to keep a straight line backwards you pick a point in front while turning your head round every few strokes to make sure you don’t collide with any thing behind. So in a similar way to going forwards you can see to correct any deviation in your course but you just have the added pressure of turning round to see whats behind you.