quote from Roland hess book page 98
Knife has another mode called midpoint. simply change the type setting in the tweak panel to midpoint.
okay… where did you hide it?
it is not clearly on the tools panel.
note to meddlers STOP messing with blender till we learn where things are now - and leave them there.
The thing to understand is that you change the operator options after you invoke the operator. The operator panel will show up after you’ve made the cut, and automatically adjust it after you change the type to midpoint.