Pressure extends a limited contact surface as an extra dimension
Aliases: force click · force touch · contact force · z-axis input
What it is
A trackpad is one plane. The finger already occupies a position, yet another command is needed that is not a tap but a harder press. Pressure as an extra input dimension means: when the contact surface cannot spare another hit region, treat normal force as a third axis that distinguishes light contact, confirm, and a deeper action. It is not a stylus pressure brush, and it is not the later question of how many levels can be held or how a baseline is taken. This layer only asks why a force channel should open on a contact that is already in use.
Why it happens
The spatial axes of a contact plane are already spent on pointing: another button either evicts a target or shrinks targets until they are hard to hit. Normal force is roughly orthogonal to tangential position, so a second intent can be coded at the same landing without moving the finger. The usual cut adds a step on top of Buxton’s three states: contact (track/preview), light press (primary), hard press (secondary). Mechanically the surface must separate command force from resting weight, or a parked hand becomes an instruction. The cost is that force has no location landmark of its own. The user must already know “you can push harder here,” and must change force while holding position—two actions that interfere.
Studying it
On the same landing, compare “place a second control in space” with “split two actions by pressure,” looking at time, errors, and whether position holds.
Independent variables: force channel on or off, target size, secondary action as a hard press versus a click elsewhere, layered haptics present or not. Dependent measures: time to fire the secondary, false primary triggers, positional drift during the hard press, whether a prompt is required to discover the press.
“Hard press is faster than a double click” misses drift: force rise micro-moves the pad, which on a small target is a mis-hit. Turning the force channel off and substituting a long-press or two-finger gesture is what tells you whether the extra dimension was worth that surface.
Where it stops holding
Stylus pressure is a different, purpose-built channel: small tip, large dynamic range; trackpad force-click findings do not move there. Gloves, fingernail contact, or a device on a soft pad drift the normal-force baseline and shut the extra dimension first. When the glass barely yields, force change is mostly pulp deformation, and many people cannot hold a stable “second step.” One-handed reaches where the thumb is already pressing hard just to land on the upper edge fold rest force into command force, and the extra axis becomes a false-trigger source. Safety-critical actions should not live only on this invisible axis.
Applying it
- Use pressure as an extra dimension only when the surface cannot host a second explicit target and the secondary action shares the same object as the primary.
- Provide layered haptics or visible yield when force changes, so “there is another step” exists in the hand, not only in a manual.
- A hard press must not walk the contact out of the original target; if it does, do not bind the secondary to force.
- Verify: on the smallest target, alternate light primary and hard secondary; record secondary hits, primary false fires, and contact displacement under load. Displacement that often exceeds the target radius means the extra dimension does not hold on that surface. Retest in the product’s common gloves and on several desk stiffnesses.