Overextension tax is applied to all teams that spend much more than they earn, on a weekly basis. The training exemption is the difference between the current overall salaries and the combined overall salaries of all players when they were acquired. Overextension Tax is not applied on the first and last economy update after the start of the season for all teams and for the first 4 weeks for new teams.
Example:
If your weekly income is -$50k, you will also pay $25k in extension tax. So you will be losing $75k/week.
But if you've also trained one player from $5k salary to $75k salary, you have a $70k training exemption. In which case your net income (for extension tax purposes) is considered to be +$20k and there is no extension tax to pay. You will only be losing $50k/week.