Freeze and reverse are the two speed operations that are not really about speed. One stops time, one runs it backwards, and both are more useful for punctuation than for effect.

What a freeze frame is for

Holding a single frame as a still image for a moment. Three jobs it does well:

Emphasis. You made a point, and the video stops on your face for half a second before continuing. It is a full stop.

An editing timeline: video clips, a zoom keyframe and an audio track below.

A place to put text. Motion competes with reading. Freezing gives on-screen text a still background to sit on, which is why the technique appears constantly in explainer videos when a definition or a number goes up.

A reveal. Freeze on the moment something appears, hold, continue.

How long

Half a second to a second and a half.

Under half a second it reads as a stutter rather than a hold, and viewers report videos like that as glitchy. Over about two seconds the viewer starts checking whether playback has stopped, which is the same failure as a completely static frame.

The exception is a freeze with substantial text on it, where the hold is as long as the text takes to read, and at that point you have made a title card rather than a freeze.

Which frame

The one just after the peak of the movement, not the peak itself.

A freeze on the exact top of a gesture often looks posed. A frame or two later, as the movement settles, reads as a natural pause. This is a small thing that makes a large difference and it costs one frame of scrubbing.

Also avoid freezing on a blink, a mid-word mouth shape, or a motion-blurred frame. A blurred still looks like a mistake because there is nothing moving to justify the blur.

Reversing, and its one big problem

Reversing plays a clip backwards. It is good for:

Loops. Forward then backward makes a clip that returns to its start, which loops seamlessly.

Undo gags. Something falls, then unfalls.

Reveals in reverse. Assembling something by playing its disassembly backwards.

And then there is the problem: audio. Reversed speech is unusable, and reversed room tone is immediately noticeable because natural sound has a shape - things start sharp and decay - and backwards it swells and cuts off, which the ear reads as wrong even when it cannot say why.

The fix: mute the reversed clip. Almost always the right answer. Let your voice or the music continue underneath from elsewhere, exactly as with b-roll.

The other reverse problem

A reversed clip of a person is uncanny in a specific way, because human movement is not time-symmetric. We accelerate and decelerate in patterns that only make sense in one direction, and played backwards a person moves in a way that reads as not-quite-human.

That is a great effect when you want it and a strange one when you did not intend it. Reversing objects, liquids and machines works far better than reversing people.

Where they sit in Zella

Slow, fast, freeze and reverse are free on both the Mac and the iPhone. Speed ramps, blade-speed and instant replay are the Pro tier.

The practical distinction is that the free tier covers changing the speed of a clip and the Pro tier covers changing it smoothly across a clip, which is the harder problem and the one that makes a slow-motion moment feel deliberate rather than switched on.

For the general case of changing playback rate, speeding up or slowing down a video covers the settings on both platforms.

The restraint argument

Both of these are noticeable effects. A freeze stops the video and a reverse breaks physics, and a viewer registers each one as a deliberate act by the editor.

That makes them expensive in attention, and worth spending on the one or two moments in a video that deserve punctuation. Three freezes in a sixty second video is already a style; five is a tic.

The test is the same one that applies to camera moves and sound effects: remove it and watch again. If the video is worse without it, keep it.

Both operations are time mapping underneath, which Apple's AVFoundation documentation describes as scaling a segment's time range, the same primitive a speed change uses.

Common questions

How long should a freeze frame hold? Half a second to a second and a half. Under half a second it reads as a stutter; over about two seconds viewers start checking whether playback stopped.

Which frame should I freeze on? The one just after the peak of a movement, not the peak itself. A freeze on the exact top of a gesture often looks posed.

Why does reversed audio sound so wrong? Natural sound starts sharp and decays. Backwards it swells then cuts off, which the ear reads as wrong even when it cannot say why. Mute reversed clips.