The Chelsea Chop is a way of manipulating the flowering times and the height of some herbaceous plants by cutting them down in Chelsea week. I've read that it came about when gardeners at the big estates tried to get everything in flower in August, the traditional month for the gentry to spend at their country abodes. By cutting back plants by about a third in late May or early June, flowering could be put back by several weeks.
It was also used to get a banking effect in a traditional border by cutting the nearest flowers to the path back by half, the middle ones by a third, and just tipping the ones at the back - works with Michaelmas daisies I'm told - but not a fashionable look now. Used on tall plants it might do away with the need for staking, and give more flower (later) by making the plant bush out.
I use the chop on alchemilla mollis in the places where I know it will block the paths by midsummer if I just leave it...it also means I get a succession of flower rather than them all going over at once - you have to put up with it looking like this for a bit though!! (I will photograph it again on the Open Garden weekend.)