Orinoco here is an example of Amy’s arm as viewed from the top.
When the arm is straight the vertex loops are squashed up at the elbow - this produces the familiar elbow concertina shape.
So on bending the elbow, the vertex loops become more even and the concertina becomes smooth. On the inside of the elbow the vertex loops are pre-stretched so on bending they become more even.
The effect is as though the arm has been modelled in a bent position in the first place - I got the idea from someone who said model the rest postion in a bent position; I did this at first then modelled it straight again by mimicking what I’d seen when I posed the bent model straight. Its easier modelling a straight model and turning the vertex loops then modelling a bent model.
Lord of the Rings Junkie shows what looks like a knee, the outside of the joint has a pointy bit which I assume is the concertina of the knee.
It takes a bit of playing around with the loops to find the best position but the result is that Amy’s arm bends without the subsurfaces overlapping.