Spath: // For him to pull the front break and send the guy over his bars to snap his neck? //
That was the front brake.
On (proper) bikes the right handlebar lever is the front brake; the left handlebar lever is the clutch and the rear brake is controlled by the right foot (left foot is the gear change).
Fortunately, they were not cornering on the limit during that part of the race - it was a relatively straight part of the track where the aim was to accelerate as fast as possible. If they had been cornering hard, there is no question that the victim would have crashed out.
The perpetrator presumably thought the effect would be to slow his rival down rather than to de-stabilise the bike too badly.
Nevertheless, the perpetrator should (IMHO) be banned from racing for life. It was such a deliberate, pre-meditated act, that the rider knew would destabilise the other bike (quite apart from cheating).
Absolutely appalling behaviour