Basically it is starting one engine by using another engines bleed offtake rather than using a APU or ground cart.
It requires the running engine to be accelerated to a level to provide enough air volume and pressure to enable a consistant start on the other engine. ( usually this level is somewhat above Flight idle ) For this reason it is best carried out on stand, or on the taxiway, and NOT during pushback, as this has a habit of Jack-Knifing the tug if the ramp surface is at all slipperly........more pronounced on wing mounted engines than tail mounted ones but even so ......
