  Under increasing compression, an unbuckled shell is in a metastable state
which becomes increasingly precarious as the buckling load is approached. So to
induce premature buckling a lateral disturbance will have to overcome a
decreasing energy barrier which reaches zero at buckling. Two archetypal
problems that exhibit a severe form of this behaviour are the
axially-compressed cylindrical shell and the externally pressurized spherical
shell. Focussing on the cylinder, a non-destructive technique was recently
proposed to estimate the 'shock sensitivity' of a laboratory specimen using a
lateral probe to measure the nonlinear load-deflection characteristic. If a
symmetry-breaking bifurcation is encountered on the path, computer simulations
showed how this can be suppressed by a controlled secondary probe. Here, we
extend our understanding by assessing in general terms how a single control can
capture remote saddle solutions: in particular how a symmetric probe could
locate an asymmetric solution. Then, more specifically, we analyse the
spherical shell with point and ring probes, to test the procedure under
challenging conditions to assess its range of applicability. Rather than a
bifurcation, the spherical shell offers the challenge of a de-stabilizing fold
(limit point) under the rigid control of the probe.
