Hand Engine is always streaming time data with hand pose information to downstream software (e.g. MoBu). The "Key to Timecode" option in Motionbuilder should adjust the timeline to the time of day in the Hand Engine recording.
In MotionBuilder, there will be an option of inserting a keyframe at the point in time defined by the timecode stamp of that data packet (Key to TimeCode) or to insert a keyframe at whatever frame the MotionBuilder scene is on when the packet is received (Key to Timeline).
Please note a really common trap when using Key to TimeCode is that the hand animation will be inserted into the timeline at a point that is often very offset from the frame counter for the take itself. When you stop the recording and remind the take to frame zero and hit play, the hands won't do anything (because the FCurves/keyframes are offset in time). The critical step is to right-click on the Action timeline in MotionBuilder and select Time --> Frame take so that it moves the timeline to where the data is. As you may surmise this mode is really useful when you are combining multiple data sources that are synced to TimeCode, and if you are playing back a raw file recorded in Hand Engine, it will play it back with the original timecode data so it stays synced in time with your other data.