Not a dumb question at all - many people on forums refer to some vaguely described clutch that they don't understand well, so this is a topic that I cover every so often so people understand what that really is a bit better.
The "slipping clutch" that people on forums refer to makes it sound very complicated, but it's not. In a typical manual winding watch, both ends of the mainspring are connected to something. The inner part wraps around and hooks to the barrel arbor, and the outer end of the spring hooks into the wall or floor of the barrel depending on the type of end on the spring. Historically there have been a large number of designs for hooking the outer end of the spring in place, so here is a chart showing a few:
In an automatic watch, the mainspring is only hooked on the inner coil to the barrel arbor - the outer end slips freely in the barrel. If you look at the mainspring from this automatic watch on my bench in front of the parts tray (Omega Cal. 1020), you will see the outer coil on the left end has another piece attached to it that forms a "Y" at the end of the spring - this is the bridle:
The spring must generate tension to provide torque, so a freely slipping mainspring inside the barrel will slip before it allows the spring to fully wind up to the proper tension. Having that extra piece of mainspring at the bridle helps create more friction between the barrel wall and the end of the spring. But that's still not enough, so we apply a special grease to the inner wall called braking grease - rather than cause less friction, it causes more, so it helps control the amount of slippage - here it is applied to the inside wall of a barrel on the same watch - this line of black grease at the red arrow:
The amount of grease here is important - too little and you will have low balance amplitude due to lack of power, and too much and you could cause excessive amplitude and that can cause things like rebanking (and a very fast rate). Many modern barrels (particularly Swatch group product) will also have small indentations on the inside wall of the barrel that helps stop the spring once it starts to slip - this prevents the slipping from going too far.
Although it's not really a common problem, if a watch is neglected, the barrel can wear on the inner wall where the spring slides. Here is a side by side shot of a brand new (left) and worn (right) Rolex Cal. 1575 barrel drum:
On the right I've added an arrow to show the groove that is worn into the barrel wall from lack of service over the years. If properly maintained, wear usually isn't a problem here, but there are always those "if it ain't broke don't fix it" guys who refuse to believe that the watch is wearing itself out if it seems to run okay, but this is the sort of thing that results from lack of servicing.
Hope this helps.
Cheers, Al
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