SpeedTree's rolling wind effect is based on a Perlin noise texture lookup. As the noise pattern sweeps through a forest, each tree's wind simulation will be excited or dampened based on which part of the noise pattern is passing over its position.
The rolling wind effect is set per compilation session in the Compiler application. In Compile settings, it's under Effects → Enable rolling wind. Note that it's a fairly expensive effect and shouldn't be used lightly. When this setting is enabled, any tree with rolling wind enabled via the SpeedTree Modeler will have the effect carry through. Those without it will remain without the effect.
The associated shader functions are found in the shader template file Include_RollingWindNoise.fx. You can see in Include_Wind.fx where calls to these functions are made, bracketed by conditionals that check ST_WIND_EFFECT_ROLLING, ST_WIND_LOD_ROLLING_FADE, and ST_ROLLING_WIND_ENABLED_DURING_COMPILE.
Rolling wind is only used in a shader if ST_WIND_LOD_FULL is also true, meaning the extra instructions won't be executed in lower-LOD wind scenarios. The rolling wind effect fades seamlessly as the full wind effect LODs to a lesser wind effect.
Example Real-Time Rolling Wind Effects