To add to what other people have said: Do this a little bit at a time - you cannot cut corners with these things and you risk damage if you try to do so. You cannot force your joints!

Also be aware that not everybody can in fact kneel for long periods at a time - various conditions cause that but I think also something to do with the length of the tendons. Long tendons (hyber mobile joints) means you feel very bendable but actually you have to be careful not to overdo it, shorter means you simply cannot bend as much as some people can.

If/when you get to see your Mistress, be sure that she is experienced and knows what she is doing with 'extreme bondage'. What they do in mags is not something you should try yourself!