Also, all this talk about ground straps may seem irrevevant, if you didn’t also read the note saying
"connect +ve’s of the battery together first,
THEN connect from the -ve of the working car to the chassis or motor of the “flat” battery car, clipping on the black (-ve) wire to the flat car LAST.
When disconnecting, Disconnect that black wire from the flat car FIRST
The reason is to prevent sparking near the battery, because the battery may be producing lots of Hydrogen gas.May. Sometimes. Of course you’ve done this 1000 times with no problem… it might someday, one in a million, explode ?
So thats why the ground strip between chassis and engine gets involved… And the brake lines may be the path the current takes…