Mass substitution is an integral process in making a time jump in the Marvel UK comics. In order to maintain the mass balance of the universe a time traveler must displace an equal mass from the time zone they are entering. Those displaced are taken out of normal space and deposited in Limbo. They remain there until the time traveler who displaced them leaves their time zone.
Bypassing the mass substitution process can have catastrophic consequences for the space-time continuum.