Tennis balls. When Pat Gilroy was Dublin boss, selector Mickey Whelan ran a drill where a tennis ball replaced a normal one in a bid to train his side to get closer in the tackle.
In Kerry, the tennis balls remain but with a different twist. Attacking players use a standard football, defenders hold a tennis ball in each hand. No more slapping and swatting, defenders have to use their full body in the tackle.
It is one of several innovative drills introduced by selector Donie Buckley, a key cog amongst the talented coaches assembled by Peter Keane upon his move into senior intercounty management.
Buckley had been with Mayo since 2012, working with both James Horan and Stephen Rochford. His contribution to their defence and tackling was oft-quoted but his brief was certainly more than that; there is no question that in recent years he was regarded as head coach with an input into all aspects.
In hindsight, Mayo forward Jason Doherty's prediction after the league final earlier this year looks remarkably astute.
There's no doubt, Donie was excellent for us and Mayo football. I have a huge amount of respect for him. We knew well that there are elements of Kerry's game that will improve further because of him. There's plenty of nights doing box-drills tackling with Donie that I'm sure the Kerry boys are going through now!
Look at their consistency and their league performances. If anything they were the most consistent team in the league in terms of the new lads they brought in and the results they got. There seems to be a huge overreaction in terms of Sunday's result for them, their trend line is firmly going up.
Buckley is a retired civil engineer and brings the same attention to detail and precise planning required there to his training. Routine trips to America are utilised to learn from other sports.
"With David Clifford or Seanie O'Shea, he doesn't want to take away from what comes naturally to them but he will focus on improving basics and decision-making," one player familiar with his methods explains. "Donie is great for a new drill. Constantly recreating yourself is what is needed in a good manager or coach. He is so enthusiastic and passionate about football. He is always trying to learn, that is why he goes to America."
The influence of other sports is by no means an original idea. Basketball coach Mark Ingle and former Irish international Jason Sherlock are both involved with Dublin. An orchestra is more than one artist and the same is true for a winning team. Modern currency lies in co-ordinating and combining different skill-sets and components under one delegation.
That is what Peter Keane does so well. Diarmuid Leen, the Killarney Legion vice-chairman, worked as Keane's selector at the club and told Balls.ie as much last year.
"He's the best communicator I've ever come across. He knows everybody inside out. He'll know the lads who are introverts and those who are extroverts; the guy that needs lots of cuddling and the guy that doesn't need much; the guy that had problems with his job. He understands everybody," Leen explained.
"He can understand what the requirements are: be it to accommodate work, strength and conditioning, be it physio, be it money."
Former players on both sides of the fence noted Buckley's fingerprints seemed all over Kerry's convincing victory against Mayo in the Super 8s. This is a man who routinely presented briefs and powerpoints on oppositions teams while involved with the Westerners. It is no surprise Keane's young guns were as well-prepped in July. Rochford still maintains there is no one better in the game at doing so than Buckley.
He has now been involved in five All-Ireland final making teams since 2011 and every time they came within a point of Dublin. Keane combined the Castleisland native with Cahersiveen's Maurice Fitzgerald and Tommy Griffin of Dingle.
They even looked outside the county for a strength and conditioning and brought in Armagh native Jason McGahan who has proved a hit with the current players. Brick by brick, they built their unit.
One man was never going to be enough to orchestrate a halt to the five in a row. It will take at least a village to topple the empire.