where the rubber meets the road
The moment of truth of something.
An athlete can train all day, but the race is where the rubber meets the road and they'll know how good they really are.
Where the rubber meets the road
where the action is most immediate, such as the place where a plan is put in motion (from the place where a vehicles tires make contact with the pavement)
If she can tell where the rubber meets the road, she's in.