Quoting DrJBHL, reply 31
Quoting CarGuy1,
reply 30
Someone has too much time on their his hands.
Jafo's asleep just now.... but I'm channeling him.
Channel away.
"Their" is the possessive case of they. "They" is the plural of he, she, or it.
There are two people in this video using their hands, hence the use of their.
"Agreement in number and gender." Sorry, CG1.
Quoting CarGuy1, reply 33... "They" is the plural of he, she, or it.
There are two people in this video using their hands, hence the use of their.
That still leaves a disagreement in number between the subject (someone, singular) and predicate (their, plural, hands).
Correct. 
I just wanna thank Doc again for posting this. I rejoined FB so I can make use of this awesome stuff!
You're 100% welcome, Jim. One can join FB and remain unharmed by staying away from the im, not playing the games and guarding high security with https browsing on the site.