What is going on with that mount unmount sequence thing is the following.
A bit long winded, but then you'll know the what, how and why.
And when you know and understand the what, how and why of mechanical things, it's easy not to break them.
First go back in your mind to the early 1950s. Electronics and electronically controlled mechanics are things still far away.
The early Hasselblads had a focal plane shutter in the body. The moving parts in the lens (the diaphragm) was of the manual type, i.e. you, not the camera had to close and open the thing. No problem there. The entire mechanism was in one place.
But when they decided to change to leaf shutters, there
was a problem that had to be solved.
With a leaf shutter in the lens of a SLR camera, the release sequence is quite complicated.
- You start with the lens wide open, so you can focus and compose. Something (a baffle shutter in the Hasselblad) has to protect the film against exposure to light while you do.
- When you decide to press the shutter release, both mirror and that baffle shutter must get away from in front of the film, but not after the shutter in the lens has closed to keep the film from being exposed. At that same time the shutter closes, the diaphragm closes too.
- When the mirror and baffle shutter has cleared the film, the shutter in the lens can open for the timed exposure. Then closes again.
- To reset the camera, the rear baffle shutter must cover the film again (which happens when you let go of the release button), the mirror must return to the viewing position, and both diaphragm and leaf shutter must open again (all these things happen, as well as winding film on to the next frame, when you turn the wind knob/crank).
This sequence of things happening in the camera and in the lens must be synchronized, so that they happen in the right order.
How to achieve that synchronisation between body and lens was the thing that needed solving.
Making it a bit more difficult was that lenses needed to be freely changeable.
And it needed an elegant solution. No bars and cogs running from camera to lens on the outside. And no tinkering to unlock complicated links between camera and lens everytime you wanted to change to another lens.
The solution they came up with was simple (when you know how to do something, it always appears simple
).
It starts with the mechanism in the lens (shutter and diaphragm). This mechanism is sprung, primed, ready to do it's business. No intricate control. Nothing fancy. Just ready and rearing to go.
It is held back however by a small catch: have a look a the rear end of a Hasselblad lens, and you'll see a tiny lever inside a semicircular surround.
You can release the mechanism by tripping that tiny catch with your finger. (Nothing bad will happen, but you will have to recock the lens afterwards.) When you do, there is no stopping it: the entire sequence runs its course.
When you mount the lens to a camera, the key on the front of the camera slots inside the slotted end of the lens' sprung drive axle.
When it is almost entirely 'there' a small pin on the camera trips the catch on the lens.
Now the key on the camera is the thing that is holding the axle, controlling the release of the mechanism in the lens.
And that's how they synchronize body with lens: at different stages of the body's release cycle the key rotates part of the way, allowing the mechanism in the lens to release only as far as is admissable. When the body takes another step, the key rotates further allowing the mechanism in the lens to proceed a bit further too, until the entire release sequence has completed with every step in the right order, perfectly synchronized between body and lens.
The bit to really remember out of that all is that only a small catch in the lens itself is keeping it from releasing, with a strong spring in the lens ready to go.
And that the camera has a pin that trips that catch.
Which it can do safely, because at the point that the catch is tripped, the camera itself, through the key, is the 'anchor' that stops the spring doing its business.
An extension tube goes between camera and lens (nothing there you didn't know).
That means it must have a key to slot inside the lens' drive axle, but also a pin to trip the catch inside the lens.
And that's where trouble looms. The extension tube is perfectly capable of unleashing the power of the spring in the lens, but usually not strong enough to act as an achor the way the camera can. (There is a similar catch and sprung axle in all but the shortest extension tubes too. But it's just not strong enough to widthstand the power of the springs in the lens.)
So what can happen when there is no camera to act as anchor at the end of the line, is that the lens and tube release in unison.
By itself no problem. You just insert a coin in the slotted axle end of the tube, and cock the entire thing, both tube and lens, again.
When you do, the thing will want to release again, so you usually need three or four hands (i make it sound worse than it is) to get the lens of the tube.
The big, only true problem occurs when the lens releases while you are actually rotating it to take it off the tube. Then, the slotted axle will turn a bit, no longer lining up with the pins and key in the mount, and it will neither come off, nor will it rotate back so you can recock the thing.
A similar thing can happen when you try to mount or remove an assembly of lens-on-tube to or from the camera.
It's rather difficult (but not impossible) to get out the fix you'll find yourself in then. So better do your best to avoid getting into it.
So always remember that the camera is the anchor, and every bit that you put on the front end of it needs to be anchored to the camera, before you can put another thing on it.
So put things on one by one, adding new things to ones that are safely connected to the camera already.
And always remember that when you are removing things, the little catch in the thing that you are removing will have to hold things. Which it will, but only reliably when all it has to hold, all it has to 'anchor', is the sprung mechanism in the bit itself is part of.
So take things off one by one, keeping the other bits you want to take off 'anchored' safely to the camera until their turn to be the outermost has come.