this is grossly over simplified ...... inside the automatic transmission you
have planetary gears inside a large drum . Around the outside of each drum
is a brake band a thin metal ring with a friction surface on the inside .
The valve body (mechanical brain) directs pressurized fluid to a hydraulic servo which tightens the brake band onto the drum . The drum stops turning and the gears then drive you . The shifts come from one hydraulic servo releasing on a drum and a second engaging the next drum , stops it from turning and the internal gears drive you forward . The transmission has an internal oil pump to make pressure ( they can wear to the point the brake bands do not hold ). The valve body looks like a maze with springs and ball bearings to direct the pressure (springs break pressure drops ) . There is also a clutch assembly with a number of friction discs ( coolant contamination delaminates them ) and a torque converter in the nose of the transmission (with internal cogs ) In short , there is a lot going on inside any automatic transmission but if they are slipping it is often a very bad thing . The drums and brake bands are not designed to slip and will quickly wear out if allowed to slip . There are a few things which can be serviced with the transmission in the car but most require removal and quickly leads to needing a rebuild . Porsche used to offer a rebuilt transmission for about $4,500 exchange . If you have a slipping transmission it needs to be looked at before it does so much damage that you need a complete rebuilt transmission but do not be surprised if there is nothing external that can be done to fix it . ( crass self promoting commercial )At which point you should consider getting a rebuilt unit such as the ones we sell for $1995 exchange . That is often the most cost effective solution rather than attempting to have your unit rebuilt as the cost of the various internal hard parts can be thousands of dollars . Which means a quote to rebuild your transmission which is open ended , Labor of say $800 plus parts could end up being a very large number but at that point in time the transmission is in pieces , the rear suspension is off the car( it can not even be towed away ). If you ask that maybe some of the used parts would be OK as the total estimated dollars are too much .... be prepared for the reply .."Well if we do not replace all the worn parts then how could it be guaranteed " . I have had a customer with car at AAMCO spend over $4,500 rebuilding an automatic .