Available Shortcuts for Proportional Edit tool:

1. O to enable Regular mode;
2. AltO to enable Connected mode;
3. ShiftO to switch among the Falloff types.

It seems that you are looking for the 3rd one. It can be functional anytime during the operation. Or you can switch from 3D View header.

Available Shortcuts for Proportional Edit tool:

1. O to enable/disable Regular mode;
2. AltO to enable/disable Connected mode;
3. ShiftO to switch among the Falloff types.