>How Remove(-1) clear the collection Class is not described anywhere.

This is incorrect.

From the VFP help file, topic "Remove method (Collection class)":
(NOTE: Bold type has been added for clarification)

Parameters
eIndex

Specifies an expression that represents a position of an item in the collection. This expression can be one of two types:
Numeric. The eIndex expression must have a value from 1 to the value of the Count property for the collection. If you pass a value of -1, Visual FoxPro removes all items in the collection.
String. The eIndex expression must correspond to the cKey that was specified for the item when it was added to the collection.

If eIndex does not match an existing member of the collection, an error occurs.