The point is as they get first team exposure other clubs will want to offer them more money than they are on here so they will be in other clubs top 25 earners thus the temptation to move is stronger.
I also agree with DaveO's interpretation but am not 100% sure how it works, but the reality is as they get more first team experience the more other clubs notice them and the more offers could potentially come in.

so is there anything to stop a team signing a player for next season and whacking him on big money as a retainer? the wage wouldnt count as he hasnt played for the first team?

Agreed !

I do think that any of the younger players coming into the team will do very well as there seems a real, air of confidence about the place that is encouraging the younger players to give it a real dig when they get there chance without fear of being completely shunned for making errors, within reason of course !

It would count if that big money put him in the top 25 earners. Being in the top 25 still counts even if they don't play. Even if, say, Fielden never plays this season, he still counts on the cap (though there are allowances made for any insurance cover, I believe).

If - and it's a big if (although a lot more likely now under the current regime) all the young lads fulfill their potential, what would the ideal pack be in 3 years? The pressure on places will be intense and someone will be squeezed out.

Props Dudson Flower Prescott Mossop Spencer Crosby Burke (all will still be young)
Back row Hock Hansen Lockers Farrell Tuson Hughes Lloyd

Presumably there's more behind this lot.

Yeah. All 4 of them :lol