method of sections truss
You do not have to crawl joint by joint just to find one bar deep inside a truss. Cut straight through with an imaginary line and solve the whole piece as a body in equilibrium, and one section often answers with a single equation. Picking only the bar you want and reading it at once is the place of the method of sections, and the rest can stay untouched.
Slide the section across the truss and the good place shows itself. A good section cuts the member you want directly, and it passes through at most three bars that still carry unknown force. Pass through four or more and the equations run short, so it will not solve in one step.
After the cut, keep one side whole as a rigid body. The cut bars become the external forces on that piece, the unknowns are at most three, and the equilibrium equations are three, so the count matches exactly. You only need one side, and the other side can wait.
The trick is one. Take moments at the point where two of the cut bars meet, those two vanish altogether, and the third member comes straight out of ΣM=0. Choosing the point so that two unknown forces pass through it is the whole of the move.
Drag the load and the effect shows at once. With the moment center fixed, the force in the bottom chord solves from a single equation, and it grows together with the load P. Choose the center well and the other unknown forces never enter the equation.
Set the two tools side by side and their characters split. The method of sections jumps to a chosen member in one equation, while the method of joints has to walk the bars one by one to get there. When you need only a few, the section is the short path, and when you need all of them, the joints are the even path.