Let's take an example.
Зашёл за угол, за которым забыл закрыть задвижку.
Some of grammarians accepted that verbs in Infinitive behave in mind as nouns.
Verb = an Action,
за (prep. or prefix) = move, slide, transit; and after still holds in a new state, denoted by that noun.
That's the key.
The actions in the example are:
шёл, был, крыть.
Шёл = he pictured as he was in the action 'walking'. Where he had to walk, is unknown, and that's the first state.
Let take a look at the noun,
угол — an abstract idea. The mind has a state, say, keeping the mind's picture of
угол, then the state moved to the more definitely state, after we're hearing
за угол, в угол, на угол.
If we say
Прошёл за угол, then we mean: he passed the corner by walking along (around) the point of the corner.
Зашёл за угол will be means: he's state of walking made a transition, and now he is already at the invisible side of the corner. And we track both states in the mind: first, he had been walked (nowhere), and now stood at the side of the corner.
Зашёл за угол за которым забыл закрыть задвижку.
зашёл = now he is already into the new state of action идти;
за угол = now he is already at the invisible side of the corner;
за которым = behind that;
забыл = now he is already into the failed state of memory;
забыл + закрыть = he's never been in the state of closing smth;
задвижка = the bolt (a metal bar that you slide across a door).