Magic imbalance is pretty straight forward. By "investing development points directly" the game means at a 1 to 1 ratio. So if (using the example in the book) you purchase a magic projection of 60 (which costs you 120 DP as a wizard/warlock) then want to do a 10 point offensive imbalance you would spend another 10 DP and end up with 70 projection in offense and 50 in defense. You can only have a maximum imbalance of 30 points in either direction and can spend 10 points per level to either further imbalance yourself (still to max of 30, that is to say in this case 90 offensive and 30 defensive) or move back toward equal offense/defense.
This is completely wrong. You do not spend DP to buy the Imbalance and you do spent DP to purchase Offensive and Defensive Projection separately.
At character creation, you spend DP to purchase your Projection ability, not Offensive Projection, not Defensive Projection, just Magic Projection, and the cost is determined based on your class. Once you have purchased the Projection you want, you can then decide to have an Imbalance, of no more than 30 in either Offensive or Defensive. This costs nothing, it is just a choice.
Eg, a Magic Projection of 60 with a 15 Offensive Imbalance would allow you to use an Offensive Projection of 75 and a Defensive Projection of 45.
a Magic Projection of 50 with a 10 Defensive Imbalance would allow you to use an Offensive Projection of 40 and a Defensive Projection of 60.
Once you start playing with that character, if you feel that the Imbalance is not right for how your character is developing then you can change the Imbalance by up to 10 points, towards either Offensive or Defensive. Again this is free, it is simply a choice.
Eg. You have a 15 Offensive Imbalance at character creation, when you next level up, you can chose to increase to towards Offensive more, up to a 25 Offensive Imbalance, or you can chose to change it towards Defensive or No Imbalance, up to 5 Offensive Imbalance. The next time you level you can again increase it either towards Offensive or Defensive by up to another 10 points.
You can never, NEVER, have an Imbalance of greater than 30 points in either Offensive or Defensive. And you can only use Imbalance if you have developed your Magic Projection using DP. You cannot, repeat CANNOT, use it if you have gained any Magic Projection from Modules.