There’s an upright pole for each bike with a sliding upside down hook on it, lower the hook over the top tube (and do the securing nut up) set up the bike and post (there’s two holes the post can slot into) in the right positions and the hook falls into the joint between top tube and seatube stopping the bike moving in any direction, or if the top tubes flat enough (road bike or MTB’s ~18″ and above) then having it in the middle of the top tube stops the bike being able to roll upwards out of the wheel supourts anyway and it’s less likley to foul the cranks or swingarm.
I go belt and braces and use some ~8″ bungee cords to hold the wheels in the rack. Or one long one from the headtube to the rear wheel suppourt to stop it going forewards (the slope of the TT means it definately can’t go back).
If that’s not clear I’ll get some pics next time I use it (probably tuesday).