Using the GPS antenna in "Normal" mode in your living room would place the boat icon on the chart....well...in your living room...and not on the water...(because that's where the satillites would recognize where the puck is...if it were able to get a signal inside your house)...
Using your unit in "Forced Simulator" mode in your living room allows use of the chart functions as if you were on the water...ie...scouting, marking waypoints, making planned routes, etc...(head unit, power supply, map card - "if desired" ....is all that is needed for use in the house)...
The physical GPS puck is not needed for this...as ITG mentioned..
You can even make recordings of the "Forced Simulator" to help in learning the recording and playback functions of the unit...(if you have an SD Card in the unit)...
Playing back this recording of the "Forced Simulator" on the unit even allows changing settings during the "playback" views ...to learn how different settings affect different views (2d/SI/DI)...