If the 'booster' is only mounted to one caliper, what real use are they? I've read elsewhere that you don't really need them :?
The booster is bolted to one side, but slips over the head of the allen bolt on the other (where Harry's got it bolted by mistake). That makes the system rigid laterally, but means you can just slip the booster back off the allen bolt head when you release the QR.
You can run them without boosters, but the boosters stiffen them up a lot. You get a much firmer lever feel with them, and a lot less boss flex.
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.