This is when the prop is on the balencer and mov it to say 10 to 4 o'clock the 10 0'clock side will start to fall.
Your prop hub is not balanced.
1) First, You should balance the prop tips with clear paint
2) After, Balance the hub with glue.
Look at the video here with Dubro prop balancer:
http://www.downonthedeck.com/videos_instruction.htm
Daniel