The method's central move is a suspension. The investigator sets aside the question of whether a religious claim is true, and also the question of what social or psychological function it serves, in order to describe what the phenomenon is like for the person who lives it. The technical term is bracketing, borrowed from philosophy, and its purpose is to prevent explanation from replacing description before description has happened.
That discipline produced real results. Rudolf Otto's account of the numinous, of an experience of the holy that is simultaneously overwhelming and attractive and is not reducible to morality, remains widely read. Later writers built extensive typologies of religious phenomena on the same basis.
The criticisms are now the mainstream position. Bracketing was said to smuggle in a theology rather than avoid one, because treating the sacred as a distinct kind of thing that must be described in its own terms is itself a substantive claim, and one many traditions would not make. Comparison across periods and cultures assembled material stripped of the history that gave it meaning, so that a rite from one century and a text from another were set side by side as instances of a type. And the essences described had a way of resembling the describer's own tradition.
The result is that few scholars now describe themselves this way, while the underlying instruction, describe before you explain, has been absorbed into ethnography.