It is not much straightforward, you must follow the formula and realize that a portion of depreciation is not eliminated when calculating FCFF from EBITDA.
NI = (EBITDA - Dep - Int)*(1 - tax rate)
Open the formula to:
NI = EBITDA*(1 - tax rate) - Dep*(1 - tax rate) - Int*(1 - tax rate)
Now calculate FCFF from NI:
FCFF = NI + Dep + Int*(1 - tax rate) - FCInv - WCInv
Replace NI with the ebitda derivation:
FCFF = EBITDA*(1-t) - Dep*(1-t) + Dep + Int*(1-t) - Int*(1-t) - FCInv - WCInv
Interests cancel each other and factorize Dep like ” -Dep*(1-t-1) = -Dep*(-t) = Dep*(t) “
FCFF = EBITDA*(1 - tax rate) + Dep*(tax rate) - FCInv - WCInv
The intuitive explanation comes from the side that depreciation reduces taxable income so you save some moeny in tax expense. That money saving is the Dep*(tax rate), therefore you add it to FCFF.
Hope this helps.