Way cool!
Benjo, my base tie...once your climbing leg is of sufficient length for the job...wrap the down leg end twice around the tree, make a bight and tie two half hitches around the down leg where it goes up the trunk, and finish with a thumb knot or other finish knot of your choice.
If you have sufficient spare rope if it were necessary to lower the climber (always supposing they are not unconscious and still clipped in with their lanyard) you just undo the knots and lower using the friction of the two wraps around the tree. I have spare rope that can be attached to the end of my climb rope if extra length were necessary.
There is a really good video on YouTube somewhere...
Caution...there is the potential for some 'sawing' motion where your rope goes over your TIP...always keep that in mind, so far I have not found it to be a serious problem, rope inspection is not showing any excessive wear...and I don't weigh much. As in any climbing system you must take into consideration your weight, your style, the tree and the job when deciding what is safe for you.