On most laptops typing on the keyboard generates accidental touches on the touchpad with the palm (usually the area below the thumb). This can lead to cursor jumps or accidental clicks.
Interference from a palm depends on the size of the touchpad and the position of the user's hand. Data from touchpads showed that almost all palm events on a Lenovo T440 happened in the left-most and right-most 5% of the touchpad. The T440 series has one of the largest touchpads, other touchpads are less affected by palm touches.
libinput enables palm detection on the edge of the touchpad. Two exclusion zones are defined (EL and ER, see the ASCII-art below). If a touch starts in the exclusion zone, it is considered a palm and the touch point is ignored. However, for fast cursor movements across the screen, it is common for a finger to start inside an exclusion zone and move rapidly across the touchpad. libinput detects such movements and avoids palm detection on such touch sequences.
In the ASCII-diagram below, the exclusion zones for left (EL) and right (ER) are marked with dots. Touch 'a' starts inside the exclusion zone and moves almost vertically. It is considered a palm and ignored for cursor movement, despite moving out of the exclusion zone.
Touch 'b' starts inside the exclusion zone but moves horizontally out of the zone. It is considered a valid touch and controls the cursor.
+--.-------------------.--+ | . . | | a. . | | |. <----------------b| | |. . | | \. . | | \ . | | .v . | +--.-------------------.--+ EL ER