In this excerpt, Charles Deitrich talked about bringing Apollo 13 back after an explosion had crippled the service module 200,000 miles from Earth.
I was the lead retro, which is the return-to-Earth guy.... I was basically responsible for the trajectory planning and recommendations for all of the return-to-Earth maneuvers, including reentry and vehicle separation activities....
There were several new techniques we had to develop to overcome situations that had never existed before. For example, for a normal entry, the command module/service module separation was controlled by the command module and thrusters on the service module. However, for this case, the service module was totally dead. I had remembered from Apollo 10 that when they jettisoned the LM [lunar module] from the command/service module, they forgot to depressurize the tunnel between the command module and LM. So when they jettisoned the LM, the air pressure vigorously forced the two vehicles apart.... I remembered this situation and decided we could use this air pressure to separate the LM from the command module. So, instead of normally bleeding the air out of the tunnel, we left some air pressure in it. First, we had to jettison the service module from the LM/command module combination. We used the LM thrusters to maneuver away from the service module. The LM thrusted forward, the service module was jettisoned, and then the LM thrusted backwards by the same amount, thus putting it back on its original trajectory, with the service module moving away.
Then later on, we used the LM to put us in the correct attitude. We vacated the LM and separated the command module from the LM by using the air pressure between the two when the docking ring was jettisoned. We had no way to use the LM thrusters for separation once the crew had gotten into the command module.