There are three springs all driven by external force with different frequency.
The resonance frequency is determined by where k is the spring constant, m is the mass (k,m can be adjusted with slider).
When sync check box is checked:
The driven frequency for left spring is ,
The driven frequency for center spring is ,
The driven frequency for right spring is .
You can find the phase between oscillation and external force.
A damping force is introduced to prevent the amplitude become infinity.