Similar to "box-seating", this is when a horse finds itself in a favourable position in the run, against the running rail and one pair of horses behind the lead.
Good horses that are one-in one-back tend to follow the leader into the race, provided they don't get caught against the rail with nowhere to make a run.
"Bob was already spending his winnings when he saw his horse sitting one-in one-back at the Valley."
