A circle, with radius 1, is centred on the origin. A point labelled 'C' rotates around the circle, starting at (1,0)
A point 'A', starting at (0,0), moves along the x axis, so that when C has completed one turn, A has moved (approx) 6.28 units.
Point 'B' is constrained to move so that its x coordinate matches A, whilst its y coordinate matches C. Thus it traces out a sine curve as A and C move.
http://www.bundy.demon.co.uk/images/sinecurve.avi 3M file, needs Windows Media Player.