Assassin’s Creed Ledge Annotations(collection of lines):
In the same way that motion while on a ladder is constrained, motion while hanging from a ledge can be constrained.
Steps to success:
have code to identify when the player “latches” to the ladder / ledge / pole / narrow beam (collision, trigger, coordinate check, whatever)
while the player is latched, their primary movement is disabled and a special move controller takes over (only allows appropriate motion with special hand-over-hand animations)
accept input and move the player along the constrained object appropriately
when the termination condition is reached, disable the special controller and revert to the default player controller
If you want ideas about implementing a linearly-constrained motion in a true 3D environment, feel free to look through my 2.5D level mover. It’s the same exact idea, except it ONLY has that motion mode 100% of the time.
This is very advanced and long topic, it is impossible to properly discussed in a forum post. But luckily SharpAccent developed a (actually two if you look for it) series about it. So I will just leave it here. 
