Double Lean is a glitch that allows the player to lean twice as far as intended. It is achieved by utilizing both the Lean key and LeanLeft or LeanRight. It is usually combined with a save and reload to allow for greater movement and clipping Daniel's view through walls. To gain full utilization of this glitch it is highly recommended to not rebind the Lean action. Sometimes the Lean action will be unbound by default. You may bind it to any key, but Alt is recommended. Double Lean may not work at all if you have a controller plugged in.
To activate Double Lean you must hold the Lean key (Alt by default), then tap the LeanLeft or LeanRight key (Q or E by default) and then move your mouse in the same direction (left for LeanLeft and right for LeanRight). In this state Daniel can walk, crouch, jump and use the lantern, but his direction is locked unless you let go of the Lean key.
If you save and reload, you take the glitch to the next level. To save and reload tap the F4 key (as you're already holding the Alt key). After a save and reload, the player's default view will be crooked in the direction you leaned in, and you will have full control over the camera as the Lean key no longer needs to be held. This allows you to perform a Lean Swipe, which can be used in Any% (Quitouts) to grab the rod in Study very quickly.
Rare Lag Bug
It's worth noting that if you have multiple keyboard inputs (US/JP, UK/US, NO/UK, FR/US etc.) you can run into an FPS stutter issue if you're not careful. The key combo LeftAlt + LeftShift changes your Windows input language if you have multiple keyboard layouts set up. Study is one of the maps where it is optimal to double lean into a quitout, while sprinting forward. In other words you'll be holding down LeftShift, W, LeftAlt and F4. Among those four keys, the combo LeftShift + LeftAlt is used, which will change the keyboard layout. This can cause the game to stutter at random when moving the camera, and makes quitout buffers no longer work.
Pressing LeftShift + LeftAlt again should fix the bug, but to prevent this entirely, all you have to do is to disable the key press combo in Windows Settings, or avoid sprinting when doing a double lean quitout.