How is it possible if the rotor "disc" (rotor blades flapping) is moving freely? How is it possible without bumping the mast?
Are you asking how does mast bumping NOT happen if the blades flap in a teetering rotorhead?
The rotor head uses "feathering" to balance the amount of flap experienced due to dissymmetry of lift while the aircraft is moving. (This doesn't happen at a static hover with no wind. No flapping occurs)
Example: The advancing blade is at a decreasing pitch, while the retreating blade is at an increasing pitch when forward cyclic is applied.
This "feathering" of the pitch balances the lift across the rotor disc, reducing flapping.
As you apply a change in cyclic, the resultant thrust in the direction of the change tilts the shaft and aircraft with it, keeping the flapping within safe margins and the gap between shaft and rotor allows some teetering prior to the fuselage changing orientation.
This all fails during low-G, and causing the rotor disc to tilt to the left when the aircraft rolls to the right does NOT pull the fuselage and shaft into the same orientation, exceeding the shaft/rotor teeter gap, causing bumping.