Several AFL coaches were increasingly frustrated with the interpretation last year, particularly in the opening half of the season, but players have been warned now they risk conceding a free kick if they attempt to do too much.

AFL operations manager Mark Evans said the umpires had been too lenient early last year.

"We have now really put into words the way we were adjudicating towards the end of last year. I thought at the start of last year, we were a little too lenient and a little inconsistent with various parts of the holding-of-the-ball rule," Evans said.

"Sometimes a player takes someone on and you can hear the crowd yell: 'That's holding the ball'. So, we have tried to capture that into how we brief umpires to pay those free kicks."

Evans also confirmed there would be a crackdown of blocking in marking contests, something AFL great Wayne Carey has already said will be hard to monitor.

While launching the NAB Challenge on Monday, Evans said Essendon's recruitment of top-up players was progressing well. The Bombers are in the process of having these players sign contracts, with about 15 or 16 to be brought in for the pre-season competition. Twenty-five players from the 2012 list, of which 17 are embroiled in the supplements saga, will not play.

"It's mostly gone OK to date. My understanding is that Essendon have already got an in-principle agreement from half a dozen players and they may look to extend that by a few more this week," Evans said.

"They will confirm with us by the end of this week about how how many players they think they will need for this early part of the NAB Challenge."

Evans said he did not know when AFL anti-doping tribunal chairman David Jones would deliver his verdict on whether 34 past and present Bombers had been given the banned drug, thymosin beta 4.

"I am assuming they will tell us when they are very close. Originally, they said they may need towards a month so we are not expecting to hear anything immediately," he said.

Evans also said the AFL was investigating its betting protocols in the wake of a police probe into a bet that former premiership player David King placed on last year's Rising Star Award winner, Lewis Taylor.

King, a prominent television commentator, placed bets of $500 and $350 with Sportsbet at the odds of $4.50 on September 1 last year. He made a profit of $2975 when Brisbane's Taylor edged the Western Bulldogs' Marcus Bontempelli by one vote.

"It's fair to say we are reviewing our procedures around the Rising Star and other awards and we will make sure we have those in place before this year's awards take place," Evans said.

The probe by Victoria Police's sports integrity unit continues. King maintains his innocence. Whispers that a Brisbane Lion would win the award emerged in the days leading into the count.