You can make depth chart changes to help you when training players. For example, if you are training someone on defense, such as a linebacker, go to the depth chart and substitute the running back for someone with a lower rating. This will make things easier when you do the lineman challenge. You can use this trick to help train almost any position.
Use this trick to build up your players in Franchise mode easier. Select a player you want to improve (for example, your defensive lineman). Then, take the slowest player on your team (for example, your kicker), go to "Player Management", and change him to a running back. Then, start him on the depth chart. When you go through the drill you will be tackling the slow kicker instead of someone such as Frank Gore.