increased piston pressure on the clutch frictions/steels leads to less slip under torque
less slip = less chance of burning the friction material off the clutches and burning many of the transmission internals when the ATF is contaminated/degraded.
I'm pretty sure its nothing more than a series of resistors with a couple stock connectors...meant to trick the computer to increase pressure from the pump/valves
45RFE - 545RFE - 68RFE Line Pressure Booster
If you check the http://www.sonnax.com/
website, it becomes pretty obvious thay are not into selling "snake-oil" ebay gadgets or 100hp chips for $49
...More pressure for more Torque...it's a no-brainer.
The 45/545RFE transmission uses a full range line pressure sensor mounted on the outside of the transmission for "closed loop" line pressure control. Due to this system, traditional methods of raising line pressure by changing the PR spring or valves have no effect. The Sonnax Line Pressure Booster alters the pressure signal sent to the computer causing the computer to raise line pressure has 15-30 psi increase. Higher line pressure increases clutch holding power and torque capacity but with these transmissions, the increase has a limited effect on shift feel. Changes in shift feel will be noticed mostly under aggressive driving or loaded conditions. This part is ideal for heavy duty and modified vehicles and even stock transmissions when a little extra pressure is desired. No other modifications are required. The Line Pressure Booster installs between the pressure sensor and vehicle harness using OEM style sealed connectors. Simply unplug the vehicle harness and reconnect with one of these parts.