Shimmer
The Shimmer group contains controls to make the light "shimmer". Here this means that the ray length is modulated by an animatable fractal so that the rays seem to move in and out and sideways.
Amount
This control sets the bias of the shimmer against the glow without shimmer. When the Amount is set to 0, no shimmer is generated no matter what the other control values are in the Shimmer section.

Detail
The detail control sets how fine the shimmer detail appears--the higher the value the more thin lines that are generated.

Shimmer Off (Amount =0)
Low Detail Shimmer
High Detail Shimmer

Phase and Loops
Phase controls the phase of shimmer. Animating this makes shimmering move randomly. To make a seamless loop, check the Use Loop checkbox and set Revolutions in Loop to the desired amount.
For example: if the loop should be 4 seconds long and consist of two whole phase revolutions then set two keyframes for Phase, one with value 0 (zero) at time 0 (zero). Then set one with value 2 at time 4 seconds. Make sure the last frame (the one at 4 seconds) is not included in the loop. If this was included there would be two identical frames following each other. Set Revolutions in Loop to 2.
With extremely high Amount (>1000) and high Detail values (>200) you can generate spikey or cloth-like patterns of lines as shown below.