When Keep In Play is ticked this will keep any unmatched bets in play.
This is similar to the standard Betfair feature on the website keep bet. It only works with In Play Horse racing markets or other In Play Events like soccer.
You could use it effectively with Tick Offset to put in offset bets that do not get matched, but then go in play to try and get matched. A sneaky trick which can be quite powerful!
You could also use this with Fixed Price Bet option.