Beast's stolen power is activated when you use 4 movement powers in 5 seconds.
Well, you may worry about the activation since movement build is little... weaker compared to usual build.
You see, there is no movement power on my skill bar.
How did I activate Bestial Frenzy? Guess what.
I just used "R" key 4 times, which uses travel power, and the travel power is movement tagged.
There's little cooldown between each "R" key use, so in practically you should use other power between "R" key.
I'm not sure if typing "R" between your dps cycle may help your overall damage.
I think it's pretty cool buff, but not tested yet.
About the duration of "Beatial Frenzy", it is affected by power duration increasing option.
In my case, the buff lasts about 12 seconds.(originally 8 seconds)
This activation may be used in Quicksilver insignia(+3 speed when you use movement power)
or Midtown ring(+20% base damage when you use movement power)