Start simulation with left switch closed and both right switches open. Operation is constant resistance (CR) mode bellow 50V supply and constant current (CC) above. If left switch is open operation is CR mode bellow 5V supply and CC mode above (but simulation won't start like that).
With the left switch closed and AC source connected on right side of schematic the load line shows ten Ohms that the wiper can step through.
With the left switch open the load line shows one Ohm up to about 5V and then becomes constant current. The POT (MCP41010) has 255 steps each adjusting the load line (and CC line) as a ratio of the initial one Ohm.
A programmable resistance like MCP41010 can only have 1mA max through its wiper.
Looking into constant voltage mode (CV) also.
MightyWatt needs CR mode (it has CC and CV):
http://kaktuscircuits.blogspot.com/2014/07/mightywatt-revison-2-now-50-mightier.html
EDN article that gave some ideas:
http://www.edn.com/design/test-and-measurement/4322585/Digitally-programmable-resistor-serves-as-test-load
