The income range where your marginal rate of alternative minimum tax (AMT), normally 26% or 28%, is temporarily boosted to a higher level. This happens under the AMT calculation because of a phaseout of the AMT income exemption that reduces the amount of income subject to the AMT. The increase in the rate happens when your additional income starts to result in a loss of the exemption amount up to the point at which it totally phases out. Additional income inside this range increases total tax liability more quickly than when income is outside the AMTI exemption phaseout zone.

This is just one type of tax bump zone.