[smogcutter]

Correct, it’s like a mirrored fan blade and rotates continuously.

Ironically, with mirror reflex system what you see through the finder are precisely the moments not captured on film. Very quick events, muzzle flashes for example, can be missed entirely.

Notably, some Bolex cameras (and others?) used a beam splitter system where you do see what the film sees, with no flicker from the shutter. The tradeoff is a dimmer image in the finder, and you’d need to overexpose to compensate for the lost light.

[actionfromafar]

Curiously, there was also a Canon SLR with a beam splitter.

https://en.wikipedia.org/wiki/Canon_Pellix

[numpad0]

Technically, many dSLRs use beam splitter principle and secondary mirrors to feed the image to AE/AF device at the bottom side of mirror box.

[RobotToaster]

Sony also made some SLR that used beamsplitters https://en.wikipedia.org/wiki/Sony_SLT_camera