The "Chelsea Chop" is so called because it is a pruning method best carried out at the end of May. Which happens to be when the RHS hold their world famous "Chelsea flower show".
The idea is to delay flowering into late summer and early autumn. Also to reduce plants so they don't become too tall and leggy. We do this to our Sedum plants but many others can be treated this way. These include Phlox, Helenium, Echinacea etc
The method is, quite literally, cut the stems down by as much as half with either secateurs or shears. Other ideas are to only cut half the plant down to allow almost two flowering seasons and if you have several clumps of the same plant then select a few to again extend the flowering season.
Feed and water the plants for a longer flowering, tidier flower bed or border.
The plant on the left is "Before" and the one on the right is "After" the Chelsea chop.