When you promote to higher league levels you become able to overclock your players.

Overclocking means adding more than the standard 8 DNA strands into a player. You can add up to 12 DNAs in total into a player when he is overclocked.

The player will naturally become better when overclocked BUT BEWARE - it comes with a cost. The players career energy will drop, so he will be able to play less matches in his career before reaching critical condition where you will have to recharge the player or eventually dismantle him.

Also overclocked players are a bit harder to recharge than normal players.

So think twice. It can be effective and create some crazy monsters for you, but is it worth the downside? Sometimes... not always. You have to make the call what is best for your team.