Do you mean 'contracting out'? If so, part of your NI contributions go to a private pension, and you end up with a smaller state pension. You still pay NI towards your pension and you still get the basic pension when you retire, but you do pay less NI. You don't get the State Second Pension that people who have not contracted out will get. This used to called SERPS.
Who knows how a private pension will fare? You could end up much better off, or much worse off.
Read here:
http://www.pensionsadvisoryservice.org.uk/Pens ion_Rights/Contracting_Out/