You can picture reciprocate as one line with movement running along it both ways. Something goes out, and then the same kind of thing comes back along the same line. A bag of oranges crosses the hallway, and a tray of egg tarts crosses back. A feeling goes towards a person, and the same feeling returns. Inside a machine, nothing crosses at all: the part itself goes out and comes back, out and back, for as long as the engine runs.
From the word’s history
Latin recīprocō meant 'to move back and forth', from reciprocus 'back and forth', possibly from a phrase like reque proque, where re- is 'back' and prō is 'forwards'.
拉丁文 recīprocō 的意思就是「來回移動」，來自 reciprocus（來回的）；再往前可能是 reque proque 這樣的說法，re- 是往回，prō 是往前。所以機械上的「往復」不是後來才長出來的用法，反而最貼近字根原本的畫面，而人跟人之間的「回報」是同一個畫面搬到人身上。
Wiktionary entry for “reciprocate”, etymology text supplied in this batch's evidence packet (source_ref kaikki:3f6f69a190036ae4:1175570597:16905)
登入就可以看每一支怎麼連回核心畫面。