NOTE: This answer is outdated. You don't need to modify the driver configuration file to enable Momentum anymore. You can simply change the relevant DWORD values at the following registry keys to enable/disable Momentum and a host of other features.
To enable Momentum, set Momentum_Enable and Momentum_Display to 1. You can find them in the registry keys listed below.
For Windows 10:
Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Elantech\SmartPadDisplay
Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Elantech\SmartPad
After doing this, Momentum or 'Inertia' related settings will start showing up in the 'Elan Smart-Pad' application that lets you configure various features of the touchpad.
I finally figured out how to enable Momentum (called Inertia in Elantech touchpads) as well as a host of other hidden features in ELAN manufactured touchpads.
Thanks to this blog post, I found out that the way to do this was to edit Elantech drivers (ETD.inf). If you're using WHQL inbox drivers, you'll probably have to downgrade to older Elantech drivers for this to work.
Make sure that the touchpad drivers you have include a setup.exe file so you can install the drivers. You'll also have to run Windows in Test Mode with integrity checks disabled so you can install the modded drivers. To restart Windows in Test Mode, run the command prompt as administrator and enter the following commands:
bcdedit -set loadoptions DISABLE_INTEGRITY_CHECKS
bcdedit -set TESTSIGNING ON
Uninstall the previous touchpad drivers and make sure you check the Delete the driver software for this device box
Now open ETD.inf in the Elantech drivers folder with a decent text editor (I recommend Notepad++) and change all instances of
Momentum_Display,%REG_DWORD%,0
to
Momentum_Display,%REG_DWORD%,1
Also change the first instance of (you'll find this in the [ETD_SmartPadUI_Default.AddReg] section)
Momentum_Enable,%REG_DWORD%,0
to
Momentum_Enable,%REG_DWORD%,1
If you'd like the cursor to bounce off of screen edges, then change:
Momentum_Bounce_Enable,%REG_DWORD%,0
to
Momentum_Bounce_Enable,%REG_DWORD%,1
There are lots of other options you can enable/disble as well. However, I'll leave it to you to experiment with them. You'll find the most interesting settings in the following sections:
[ETD_SmartPadDisplay_Default.AddReg]
[ETD_SmartPadDisplay_Win8.AddReg]
[ETD_SmartPadUI_Default.AddReg]
WARNING: Make sure you know exactly what you're doing here or you could end up with an unstable driver and lots of BSODs. I recommend making a backup of the original drivers before you start to modify them.
After you've finished editing the drivers, reboot your computer and install the new modded drivers by running setup.exe
Best Answer
Most of the time the Synaptics driver isn't the issue - it's some other process disrupting the driver. To find the culprit process, try going to the Resource Monitor to see which processes are running when this issue is occurring. Otherwise, I'd recommend you to look up Synaptics touchpad troubleshooting.
Anyhow, since Synaptics drivers are rarely the problem there are very few alternatives. One of them, though, is two-finger-scroll. Note that this application's last commit dates back to February 2009 so there might be issues with this as well.