Kneeling on pencils

[yub]

I was suggested to try kneeling on pencils. One pencil under both knees, and the pain feels unendurable. With the permission of my Mistress i tested it. Of course with bare knees and the pencils on the bare wooden floor. I found as i were told, the pain seemed unendurable in the first minute. Then it dulled a bit, remaining severe but not extreme. I have persisted for 15 minutes and could have endured more, but wanted to remain on the secure side. After getting up there were deep and hard delves on my knees, which lasted for several hours, and my knees were sore on the following day. Nothing more happened.

My Mistress says she will employ this punishment on me if she makes sure that it doesn't cause lasting damages. My question is: are you familiar with it? Does it cause serious harm? How long can it be done securely? Thank you for every answer.