How does this latch relay work?
The diagram shown is for an AC-coil (or AC-DC) latching relay. The series diodes are there to rectify the AC. They're typically rated at something like 1kV PIV. 

The resistor is for "ampere-turn compensation". Generally the reset coil needs much less current than the set coil. 

For higher voltage relays (>50VAC) it may be present inside the relay. For lower coil voltage relays it may not be present (just a coil). 

It's more of a problem with high voltage relays because the wire would have to be very fine to get the optimal resistance so it's easier to just throw away some of the power in a resistor.