When Lara gets on a very steep slope, she looses her footing and starts sliding down. In the classic games was unable to steer while sliding. To change the lane, one has to jump up and steer. In Tomb Raider (2013) the player has to steer her left or right to avoid hitting obstacles in her way while sliding. Sometimes, to avoid death by falling into an Abyss or a Spike Trap, Lara has to jump at the end of a slope.
Sliding while Running
To slide under obstacles in Lara Croft: Relic Run one has to swipe down on the Touchscreen. Lara will get on her knees and slide along the floor. She can still switch Lanes while sliding by swiping left or right, but she can't Jump. The slide move only lasts for a few seconds; Lara will automatically get up after that.