Without any modifiers, ~_ is the same as (pprint-newline :linear). ~@_ is the same as (pprint-newline :miser). ~:_ is the same as (pprint-newline :fill). ~:@_ is the same as (pprint-newline :mandatory).
~_
(pprint-newline :linear)
~@_
(pprint-newline :miser)
~:_
(pprint-newline :fill)
~:@_
(pprint-newline :mandatory)