The idea is to avoid potentially controversial situations when play continues only to be later stopped because the referee had not immediately noticed the linesman's signal. When the flag is raised and goes unseen the linesman presses a button on the handle to activate a radio bleep in the referee's top pocket.
While cricket has embraced new technology with the introduction of a third umpire watching television replays to rule on hairline decisions and tennis has used the Cyclops machine to aid line calls, football has been reluctant to follow the path to mechanically aided enlightenment.
A Sky TV request to have referees wired for sound enabling commentators to have contentious decisions immediately explained was rejected by the Football Association at the start of the season. It was receptive, though, to a subsequent suggestion that adapted flags might help.
They were first used in the Wimbledon v Colchester FA Cup tie in January and also in a recent Crystal Palace fixture. However the FA's director of refeering, Ken Ridden, says the evidence so far is unconvincing.
"In the games where it has been tried there has not really been an instant where the referee has been unaware that the flag has gone up. We don't really see this as a major problem in the game - it tends to be of more concern lower down in the junior leagues - but anything that can help is useful.
"For instance I was at this week's Uefa Cup semi-final between Juventus and Borussia Dortmund when the ball ended up in the net but the goal was then ruled out because the linesman was flagging that the ball had gone out of play."
More experiments are planned and although the cost could spiral to £20,000 to equip every linesman in the professional game, Ridden said it need not be prohibitive.
n Ken Bates, the Chelsea chairman, has denied speculation, following Thursday night's European exit, about a rift with the club's manager, Glenn Hoddle, who has announced his intention to stop playing at the end of the season. Bates also denied suggestions about a power struggle with Matthew Harding, the multi-millionaire director, who has just bought out the company that owns Stamford Bridge and its liability for a £16.5m bank loan.