The 24-year-old – who can’t be identified to protect her children’s identities – said she’d left the young boy and his five-year-old brother outside for just a few moments before the drama.
She came back down to the kitchen and saw the “stocky” stranger with a “circular face” lean over the wooden fence and lift her son up by the armpits while her other son tried to get out through the gate.
She said she saw the younger boy in the stranger’s arms on the pathway which runs along her friend’s back garden in April last year.
The man then walked away at a “normal” pace carrying the boy with his right arm wrapped around the youngster’s waist.
She said: “I froze for a minute then I ran out the back door. I threw myself over the fence. It was pretty fast.”
She said she shouted out her son’s name twice and the man dropped the youngster.
“My heart nearly stopped,” she said.
“Except for lifting the wain and going back into (my friend’s) there wasn’t really a lot I could do. My son had been dropped and (the man) had ran, so he was out of sight within seconds.”
She said she got to within six feet of her boy’s captor, close enough to get a good look at his face.
She added: “He turned round before he dropped him. There was still light but it was getting pretty dark. My son was fine. I held him. I took both boys back inside.
“I went and told (my friend): ‘A guy’s just lifted my wain from the back garden’. She ran out. I stayed indoors with the three boys.”
When her friend returned 10 minutes later they dialled 999 and police arrived quickly on the scene.
The mother, who has since moved to live in Grangemouth, first identified a suspect from a book of police photographs, then picked out the same man at an identity parade at Falkirk Police Station.
Asked if she recognised the man in court she pointed out the accused Derek Hornall, 40, a prisoner at Barlinnie in Glasgow.
Hornall denies reaching over the garden fence, seizing the child under the arms, lifting him over the fence and carrying him away and detaining him against his will on April 25, 2015.
He alse denies behaving in a threatening and abusive manner at another address in Bonnybridge on October 5, 2015 by chasing and threatening to kidnap a nine-year-old girl and two boys aged seven and 11 as well as possessing cannabis at Falkirk Police office the same day.
The trial, before Lord Woolman, continues at Livingston Sheriff Court.