The striker unleashed an unstoppable shot from the edge of the box after Michael Carrick cleverly picked him out by squaring a free-kick - most people were expecting a shot from Reto Ziegler.
That made it 2-1 just four minutes into the second half and after trailing 1-0, the turn around was complete five minutes later when Jermain buried his second goal from Fredi Kanoute's cross to register his 19th of the season.
Head Coach Martin reflected: "I was very pleased with the free-kick because we tried it in training and it came off, that is always nice to see. It was a terrific shot as well but Jermain is a terrific finisher.
"Michael Carrick waited to create the moment, made eye contact with Jermain and it worked.
"You can do everything in training but it's about finishing and if you have a finisher like Jermain, free-kicks like that can be successful."
Martin pointed out that Jermain's double arrived via an improved second half display.
"He had a quiet first half and then we played better and that's the story," he added.
"If you play better there is better service to the strikers and they score.
"Robbie Keane scored a terrific goal when he came on last week against Portsmouth, he also scored the winner against Brighton, now Jermain has done it as well."
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Martin Jol insisted free-kick routines like the one that sunk West Brom on Saturday are always possible with someone like Jermain Defoe around.