I recorded a video to best explain this, but reproduction is pretty simple:

• Grapple onto something, pause the game, wait a bit and unpause. You'll have gained a lot of momentum (enough to do full circular swings).
• Grapple onto something (like an inverted L) and then catch yourself behind the pillar, you'll slowly move downwards and gain more momentum, move out of the way of the obstruction to gain a lot of speed.

Video: http://dl.dropbox.com/u/14141289/momentum.mp4 - 3m4s, 141.7MB

Mod infos:
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