Going into the match, West Brom were bottom of the Premier League's form table, and Foster believes that the win could be a turning point in their season.
"We needed that. We have been on a poor run. The performances have been decent but we knew we had a chance. We knew set pieces would be a big one for us," said Foster.
"Dean Kiely, our goalkeeping coach, does some research [on penalties] and he told me to look to my left. With that in mind, and with a little bit of my instincts, I went there and managed to save it.
"I hope this is a turning point in our season. This puts us on 36 points so we are in touching distance of the 40-point mark where you can feel safe from relegation. If you had said we would be above Liverpool at this stage of the season back in the summer we would have taken that."
The result lifts West Brom up to eighth in the Premier League table, one point ahead of Liverpool.