How could a collision have helped Neptune capture Triton?

It is thought that Triton was originally an independent icy planetesimal that formed elsewhere in the solar system before being captured by Neptune during the planet's early history.1

A collision with an existing moon of Neptune could have helped facilitate this capture by slowing Triton down sufficiently. This reduction in velocity would have allowed the planet's gravity to place Triton into an elongated, retrograde, and highly inclined orbit. Following this initial capture, tidal interactions between the two bodies would have gradually circularized Triton's path over a period that may have lasted more than one billion years.1