The DC biasing of the transistor is determined by R6 and P4.
Bias adjustment by P4 allows the amplitube of the generated wave to be adjusted.
Moving P4 to low values means shifting the transistor bias nearer to cutoff and also lowering its gain.
This means that the lower is P4,the smaller is the delivered oscillation amplitude,to the point where oscillations are extinguished.
On the other hand,raising P4 means an increase in gain and a larger swing for the generated signal,to a point where loss of linearity progressively occurs.