The lock up point is where the rod is loaded and "locked" up as far as it can bend. Then the power is applied and stored until the release. A bit like my avatar pic where my rod is at its lock up point as I apply power into the cast.
Thank you Luke for this diagram and also Mick for the ones he has been putting up. I think these diagrams really help beginners get their heads right about what is happening during a basic OTG cast. Seeing it in both overhaed and 3/4 view makes it much easier to comprehend to a first timer I think.