What Aaron_Ewing's talking about is an SA-MP bug that allows you to jump about three times the height you gain with a normal BMX jump, and isn't allowed for use against players to have an advantage over them, I could further explain the the bug, but that would encourage players to try it, and who knows if they'll be able to resist the urge to try it during serious gameplay to gain an advantage.
If the player was seen shooting during the jump (from the bike), right when he jumps, or in mid-air, he's using the bug; if not, he has modifications installed. I can't say for sure if the height shown in the screenshot Marwin captured can be achieved using the bug, I don't remember very well how high it could let one jump. But like I said, the bug can't be used without shooting during the jump.