Check out how Jorge C uses rest strokes for the melody and how he barely has to move the fingers out of position to make it happen. What he does is he collapses the tip joint. It's my belief that free stroke and rest stroke movement is the same other than this tip joint collapsing more often for rest strokes. His hand shows how it's the middle joint flexion while the tip joint is relaxed that is largely responsible for the pluck for both strokes. The main knuckle helps with some effort but the tip only collapses when the middle joint flexes. If I practice alternating i,m with say 4 notes free and then 4 notes rest repeatedly, and while doing so I really exaggerate the middle joint follow through, it greatly relaxes how my rest strokes feel because I'm not attempting to make the rest strokes heavier (ie using more large knuckle joint to push through the string). Letting the tips bend backwards helps too but it's really mostly about that feeling of generating the pluck from the middle joint movement that makes my rest strokes feel light and easy.