When you are hanging from a ledge, a Grip Gauge appears on screen. By doing a ton of pull-ups (press L2 and R2 simultanously), you can increase the Grip Gauge and increase the alotted time that you are able to hang from a rail.

Level	How to unlock
Grip Gauge Level 2	Do 100 Pull-ups.
Grip Gauge Level 3	Do 100 Pull-ups after unlocking Grip Gauge Level 2.