A TIRZ is created to capture additional tax collections per year in a defined area and dedicate the additional tax to capital/public works improvements within the zone. For example, if the tax at the beginning is $1M, and the tax after year one is $1.5M, the the TIRZ would receive $0.5M for the year.
According to the TIRZ documentation, "The proposed Medical Center of the Americas (MCA) Tax Increment Reinvestment Zone is intended to stimulate private investment in the El Paso MCA area and facilitate the creation of an integrated campus of facilities that will position the MCA as the premier regional center of health delivery, education, and research. The full implementation of the proposed Zone is expected to yield significant increases in tax revenues for all MCA area taxing authorities, providing substantial economic benefits and opportunities to the region."

City Council Agenda item (PDF): http://www.elpasotexas.gov/muni_clerk/agenda/05-22-12/05221210A.pdf
MCA Master Plan Details: http://elpasodevnews.blogspot.com/p/master-plans.html