On: 'Underwater bicycle' propels swimmers forward at s...

[404mm]

Balancing this device must have been a good problem to solve! All I can think of is “where does the other end that stick go?”

[a_c → 404mm]

Two propellers spin in different directions might solve it I guess

[4gotunameagain → a_c]

That will counter the rotational force, but not the transversal up your butt.

[abecedarius → 4gotunameagain]

Maybe design a pogo-stick-like action, pushing with both legs together?

[whamlastxmas → 404mm]

Your blow hole, clearly