Courtesy of Ultimatecarpage.com, here's some modern photos of the 1982 McLaren MP4/1B, clearly showing how the exhausts were used to blow the trailing edge of the extended diffuser channels.
Note also the rocker-arm suspension, rendered largely obsolete by the end of that year, (for the front suspension at least), the rockers reportedly behaving as undamped springs under the large, ground-effect loadings of the era (Doug Nye, Autocourse 1982 Technical Review).
Meanwhile, the final photo here shows John Barnard's previous car, the 1979 Chaparral 2K Indycar. You can clearly see where the extended diffuser tunnels come from, but the use of the exhausts to blow those diffusers seems to have been an innovation on the McLaren.
For those puzzling over the plumbing here of the turbocharged Cosworth DFX engine, the turbine/compressor is the unit immediately behind the engine. The ambient pressure air inlet is the 'sucker' on the left, whilst the exhaust pipe itself is the large bore unit on the right. The exhausts from each bank of cylinders blend into a single pipe, then immediately bifurcate, one branch feeding the turbine unit, the other feeding the wastegate, which sits between the diffuser tunnels at the rear. When the compressed air reaches some threshold level, and the exhaust gases are not required to maintain boost, a spring opens a valve in the wastegate, and the exhaust gases bypass the turbine, exiting to the atmosphere via the smaller pipes either side of the wastegate.