AWHP - defrost heating strips

[desA]

I've noticed two rows of electrical heating strips positioned in evaporator coils of air-to-water heat-pumps destined for colder climates.

Can someone explain how these electric strips are operated in regards to the defrost cycle? How does the defrost cycle work for AWHP's?

With this kind of AWHP, there was no reversing valve evident (at least from what I observed). The electrical heating strips seem to be standard on all their heat-pumps - even S.E. Asian conditions where frost very seldom, if ever, occurs (except the very cold climes of northern Asia.)

What are simple, inexpensive ways in which to initiate & terminate a defrost cycle, for an AWHP? Links & literature would be gratefully appreciated.

Thanks for your applied wisdom.