Vertical motion maps to zoom in or out, sparing a second finger

What it is

Once drag-zoom is armed, vertical travel maps to scale: commonly drag up to zoom in and down to zoom out (some stacks invert). Horizontal travel is dropped or used as a slight pan of the anchor. The point is to let one thumb, from a holding posture, run continuous zoom without reserving space for a second finger sliding the opposite way. It is a pinch substitute on a one-finger channel, not pinch renamed.

Why it happens

Pinch uses the rate of change of inter-finger span as gain, which wants a palm or a second hand. One-handed on a large phone, the second finger cannot make a stable opposing stroke. Rewriting gain onto the thumb that is already moving—onto the screen’s vertical track—makes the control surface large enough at once. The cost is a lost pan degree of freedom: during drag-zoom the map cannot translate freely the way a pinch can, unless X is sent to pan on its own. The anchor is usually the second down, so scale opens around the thumb rather than jumping about the screen centre.

Studying it

Compare pinch and double-tap-drag under one-handed hold, two-handed hold, and a landscape tablet. Measure zoom time, overshoot, and whether the other hand is forced to join. Also measure final scale error and anchor drift. Contrast “up zooms in” with “down zooms in” against reading direction and the scroll-direction model. Do not test only on a desk-bound tablet used with two hands.

Where it stops holding

On a tablet already easy to pinch with two hands, the mapping is a spare accelerator. A drawing app that treats vertical drag as a stroke cannot also levy that axis. Sending X to pan will fight nested scrollers. Direction is not universal: a zoom axis that opposes content-centric scrolling will be inverted on first use. Mapping does not solve discoverability.

Applying it

• Anchor zoom at the second down; map vertical travel to scale continuously; ignore X or use it only for a small pan.
• Tune gain on a phone held in one hand so the thumb’s rest position to the top edge covers the usual scale range.
• Ask someone to take a map from street to city and back one-handed; if a second hand is required, gain or anchor is wrong—not “they cannot pinch.”

Related

• Same group: C3.26.1 The gesture is a second down that does not lift, then a vertical drag · C3.26.3 The gesture is almost undiscoverable without teaching · C3.26.4 Maps use it as a one-handed stand-in for pinch
• Adjacent: C3.09 Spread-to-zoom in · C3.22 Scroll direction mapping
• Search: one-finger zoom · vertical zoom mapping · thumb zoom