r next y value is less negative (it can even be positive)!  
But now B * sin(y) is also less negative, so
the phase increment lurches us forward, and y is now even more negative.
We've started to zig-zag down the sine curve.  Depending on the index, this bouncing
can reach any amplitude, and start anywhere after the high point of the curve.  Eventually,
the sine slope lessens (as it reaches its bottom), the overall phase catches up, and
the bouncing stops for that cycle. 
The noise is not chaos (in the sens