Drills are used to increase a player's rating.
Teams start out with 10 drills (meaning they can drill 10 different players per day). Every time you upgrade your arena you get an extra drill. So once you get to the level 5 arena, for example, you'd have 4 extra drills. Donators get 5 extra drills. Having more drills is beneficial for teams with lots of developing young players. If a team does not have a lot of prospects, having more drills might not necessarily equate to higher ratings since drills are not as effective on higher-rated, older players.
If you have more players you'd like to train than drills, you can always bet drills through shinny games. To do this, visit various team profiles and click "shinny" until you find a team that displays the "Play Betting 1 Drill" button right below the normal "Play" button. If you are having trouble finding a team that lets you bet drills against them, try going to the shinny page of higher power ranked teams. It is up to the game to decide which teams you can place drill bets with. Any drills won through a shinny game must be used the day they are won or else they will be lost.
How Ratings Fluctuate
Ratings can fluctuate via 2 methods: increases through drills and increases through the strength and conditioning coordinator.
The Strength and Conditioning Coordinator can only increase a player's rating back to their original rating. This staff member works by increasing a player's rating one point at a time and can be used on a player only once every 5 days. It costs credits to use this service, but is generally well worth it.
Natural decreases may occur upon login for players aged 32+ (skaters have a 3.33% chance of decreasing each day and goalies have a 0.91% chance of decreasing each day).
Once a player reaches the age of 36, their chances of receiving a natural decrease is greater (skaters and goalies have a 5.26% chance of decreasing each day).