Not entirely certain if this is the appropriate place, but there's a conflict between this and the Smart Moving mod.

Smart moving adds extra functionality to the sneak and sprint buttons, allowing climbing, crawling, sliding and more. When both mods are loaded it's not possible to adjust your grappling hook line length using Sneak + Forward/Backward.

MCP: 9.05
FML: 8.0.99.99
Minecraft Forge: 11.14.3.1450
Player API: 1.4
Smart Core: 1.2.1
grapplemod: 1.0
Smart Moving: 16.2
Smart Render: 2.1

• Bold mods are part of Smart Moving.

Fix from daa141d confirmed.