Pink bar is pivoted on the base.

The groove profile of blue slider ensures that the upper pink pin moves cloclwise along heart-shaped groove when the blue button is pushed and released.

Pay attention to the followings (the letters on the sketch denote corners of the profiles):

a’ is on the right of a

b’ is on the left of b

c’ is on the right of c

d’ is on the left of d

See related videos:

How a Click Pen Works

Retractable Pen Mechanism:

See another mechanism of the same working principle

“Push-push button 1”