You would half to have a object at the bottom of the map that has a physics driver for sliding or known as ice. You can also add the variable I think it is /set _friction 0 and that will make you slide anywhere and everywhere on the map.
I'd go with _friction. One of my favorite things to do on a map is turn it into a skating rink of death, by adding snow and mirror effects, then setting friction between 1.5-6.0 and increasing tank speed.
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