The eraser end is recognised by flipping the pen, so erase needs no toolbar switch

What it is

Some pens put a second contact on the tail that the digitizer can tell from the nib. People flip the pen to erase, like a pencil eraser, without tapping an eraser icon or holding a side button. Recognition is pose or a second resonator/electrode, not the app guessing that “this stroke wants to erase”.

Why it happens

EMR pens often use a second resonant circuit in the tail, at a different frequency or coil from the nib, so the tablet knows which end is in range. Other designs use an IMU to see inversion. Once “eraser contact” is reported, the ink engine routes the path to erase: delete strokes along the path, pixel-erase a raster, or hit-test objects. Flip is a large motion, harder to trigger by accident than a side-button click, and more visible—onlookers see the pen invert. The cost is time: flip, aim, flip back is heavy for one dot; a momentary barrel eraser fits that case. The two ends usually differ in geometry; the eraser is blunter, so the hit region is larger, which helps aiming at a line and risks neighbours. A pen without a second resonator cannot offer a true flip; software that treats “upside down” as eraser will fight ordinary barrel roll.

Studying it

Compare toolbar eraser, momentary barrel eraser, and flip eraser. Mix many tiny erases with one large-area erase.

Independent variables: erase entry, eraser-end width, vector strokes versus pixels. Dependent measures: time to enter erase, neighbour erasures, flip count, whether people still bother to flip for a short erase.

Log whether people abandon flip for the barrel on short erases; that decides whether the eraser end is a “large-erase tool” or the only entry.

Where it stops holding

Short pens, or a cap-as-eraser with no sensor in the cap, send no flip signal. Left- and right-hand flip paths differ, but recognition is which end is present, independent of handedness. If the eraser end is a blender (not delete), the pencil metaphor disappoints. Children may treat both ends as nibs; a tail contact then erases by surprise.

Applying it

• If there is an eraser end, trust the hardware signal; do not guess from inversion angle. If there is no hardware, do not fake flip-erase.
• Position flip as a large or deliberate erase posture; keep short erase on a barrel button or toolbar.
• Make erase undoable so a blunt miss on a neighbour can come back at once.
• How to check: after a flip, the path should only erase, not draw. Flip back and the previous tool should return at once, not stay in eraser. On a short-erase task, count how many people prefer the barrel. Report neighbour erasures together with eraser-end width.

Related

• Same group: C5.10.1 Barrel buttons map to tool switch or menu · C5.10.3 Button placement must miss the natural finger rest · C5.10.4 Button bindings differ by app, with no shared convention
• Adjacent: C5.06 Pen Gestures and Marking Menus · C5.02 Pen Tilt and Azimuth
• Search: eraser end · stylus invert · hardware eraser