From IRS Pub 946: Special (Bonus) Depreciation:
Bonus depreciation is "a special depreciation allowance under IRS rules that allows you to recover part of the cost of qualified property, placed in service during the tax year. The allowance applies only for the first year you place the property in service. For qualified property placed in service in 2015, you can take an additional 50% special allowance. The allowance is an additional deduction you can take after any section 179 deduction and before you figure regular depreciation under MACRS for the year you place the property in service."
The Special Depreciation Allowance gives you 50% of that deduction in the first year, then the other 50% is depreciated as usual.
TurboTax will offer this bonus depreciation during the asset interview sequence if your property qualifies for this special allowance. Because this type of depreciation can be "recaptured" as ordinary income when you sell the asset, taking this additional deduction is optional.
