See an attempt to explain Rashi, using a gemara in Shabbos 42a:
https://gemaraboards.forumotion.com/t106-shabbos-42a-davar-sheino-miskavein-and-melakhah-sheinah-tzricha-l-gufa#218
https://gemaraboards.forumotion.com/t106-shabbos-42a-davar-sheino-miskavein-and-melakhah-sheinah-tzricha-l-gufa#218