A Lasting Power of Attorney (LPA) only takes effect once it has been registered with the Office of the Public Guardian (OPG).
The document can be registered at any time, whether immediately after it has been completed (in which case it is simply stored safely and only put into use once it is needed) or at some time in the future when the need to use it actually arises.
Registration in itself does not mean that the individual in question has lost capacity. They can carry on making decisions in the usual way, despite registration, until such time as capacity is lost. The important thing to remember is that an LPA cannot be used at all until it is registered.