"I've just faced him up, come inside, faked to shoot and dragged it inside of him," Barnes told BBC WM.
"He's tried to turn with me and his knee's caught my knee. So that's a penalty," the 21-year-old insisted.
Swansea's players and manager Paula Sousa were furious with the decision made by referee Roger East, suggesting that Barnes should have been booked.
It is the second time in successive games that he has earned a crucial penalty, Graham Dorrans successfully converting another spot-kick against Blackpool on Saturday.
"Obviously it's good to come on and have an impact, but if I hadn't come on there's plenty of talent on the bench and plenty of talent out on the pitch to have done the same thing," he noted.
Barnes' substitute appearances have helped West Brom to three successive victories in the Championship, opening up an eight-point gap on third-placed Nottingham Forest in the promotion race.
Source: BBC Sport
Source: BBC Sport