Runner On First, Ball Is Hit...
First base runner is in a force situation---this means that the second baseman needs only to have control of the ball and contact with the base for the "force out".
If the second baseman is not on the base but more in a fielding position for a tag, when a fielding player is in a force-out situation and is in a tagging mode the base runner has the following options.
Move towards the next base... they can slow and even stop with the fielding player having to move towards the runner to make the tag.
If the base runner makes a retreating (backwards motion) they can be called out IMMEDIATELY by the umpire which then gives the fielding player the opportunity to throw to first for the attempted double. When this occurs, the umpire should call "OUT" audibly/loudly so that all hear the outcome call (even though a physical tag has not transpired.)
This rule was changed a couple of years ago so not everyone is aware of it and when this situation results clarification is usually required.