ECHO is a scheme of progressive handicapping designed in Ireland. It aims to improve the fairness of handicaps by gradually altering them based on each boat's performance. Details are available from Irish Sailing.
HalSail implements this by calculating a new handicap for each boat after every race in a series for any class whose handicap type is ECHO. The new handicap is used for the next race in the series, whereupon another new handicap is calculated for the race after that, and so on. Note that to generate new handicaps there must be at least three finishers. If a race has only one or two finishers the handicaps will remain unaltered for the next race.
ECHO allows users to vary the severity of the handicap alterations between races. There is a parameter known as the ECHO Blend which you can set when editing the details of an ECHO series. The blend is the ratio of the handicap actually achieved in a race to the starting handicap to use when calculating the handicap to use for the next race. So if the ECHO Blend is zero, none of the achieved handicap will be used and so there will be no handicap changes during the series. If the ECHO BLend is 1.0, all the change will be used in each race, which would be very severe. Typical values for the blend are between 0.1 and 0.4, with 0.3 (which is the same a that used in NHC) being the most common.
The detailed maths are given in another FAQ.
The handicaps that are calculated during an ECHO series are not entered into the Boat Register like other handicaps, they are recalculated every time the results are needed and then discarded. Any handicaps in the Boat Register valid after the start of the first race in the series are ignored. The output of the result of any race in the series shows the handicap used in that race and the handicap to be used in the next race.
As stated above, the handicaps that are applied to each race in a series are not stored in the Boat Register. They are calculated each time they are needed and then discarded. However you may want to carry over the handicaps from the end of one series to start the next series. You do this from the analyse results page. Go to Results / Analyse handicaps and select the series in question. At the bottom of the column giving the analysis of each race there is a button labeled Save handicaps. Click this link to store the handicaps calculated for each boat at the end of that race permanently in the Boat register. You will be asked when you want the realigned handicaps to start. Choose a time just before the start of the first race of the next series.
As well as storing handicaps at the end of a series for use in the next one, you may want to take a snapshot of the current handicaps to use in another racing class. Perhaps you want to run a championship with non-progressive handicaps, taken from the current state of an ECHO series. You can do that by setting up a plain TCF racing class with the boats that will be taking part in the championship, then using the above method to save the handicaps after any race in an ECHO series, putting them into the boat register for your new class not the ECHO class.