  Our goal is to reveal how the $\alpha$ cluster amplitude is probed through
$\alpha$ knockout reactions depending on reaction conditions, e.g., the
incident energy. We consider $^{20}$Ne($p$,$p\alpha$)$^{16}$O and
$^{120}$Sn($p$,$p\alpha$)$^{116}$Cd at 100-400 MeV within the distorted wave
impulse approximation (DWIA) framework. We introduce a masking function which
shows how the reaction amplitude in the nuclear interior is suppressed and
defines the probed region of the $\alpha$ cluster wave function. It is clearly
shown by means of the masking function that the $\alpha$ knockout reaction
probes the $\alpha$ cluster amplitude in the nuclear surface region, which is
the direct measure of well-developed $\alpha$ cluster states. A simplified form
of the masking function is introduced and the incident energy dependence of the
masking effect is investigated. $\alpha$ knockout reaction can probe the
$\alpha$ cluster amplitude in the nuclear surface region by choosing proper
kinematics owing to the masking effect originated from absorptions of
distorting potentials, and is a suitable method to investigate how $\alpha$
cluster states are spatially developed.
