Wheelbarrowing and porpoising are very different things. In
wheelbarrowing, insufficient back pressure leads to directional
instability. A fairly common response is to release even more back
pressure and concentrate on trying to steer, which immediately makes
things worse as the airplane swerves sown the runway in a series of
overcorrections.

In porpoising, the aircraft's nosewheel hits, causing it to bounce in
the air, drop to the runway, and repeat. This is often a divergent
oscillation -- the amplitude increases with each bounce until either
back pressure is applied or the energy decreases sufficiently. It
usually happens on touchdown.

A nosewheel landing usually results in a porpoise. Releasing back
pressure on rollout (often in crosswinds) is the normal way to initiate
wheelbarrowing. On Cessnas, retracting flaps for a touch and go
without adding back pressure can start the wheelbarrow.

Paul