  Accretion is an essential physical process in black-hole X-ray binaries
(BHXRBs) and active galactic nuclei. The properties of accretion flows and
their radiation are generally considered to be uniquely determined by the mass
accretion rate of the disk; however, the "hysteresis effect" observed during
outbursts of nearly all BHXRBs seriously challenges this paradigm. The
hysteresis effect is referred to that apparently similar spectral state
transitions take place at very different luminosities during an outburst cycle.
Phenomenologically, this effect is also represented as the so-called "q"-shaped
hardness-intensity diagram (HID), which has been proposed as a unified scene
for BHXRBs. However, even with the high cadence pointing observations, the
distinctly important rapid-rise stage has been caught to date in only a few
BHXRB outbursts, and it is still limited to narrow energy/wavelength bands. As
a result, there is still lack of a quantitative theoretical interpretation and
observational understanding on the "q"-diagram. Here, we present a detailed
time-lag analysis on Insight-HXMT's intensive monitoring data of a newly found
BHXRB, MAXI J1348-630, over a broad energy band (1--150 keV). We find the first
observational evidence that the observed time-lag between radiations of the
accretion disk and the corona leads naturally to the hysteresis effect and the
"q"-diagram. Moreover, complemented by the quasi-simultaneous Swift data, we
achieve a panorama of the accretion flow: the hard X-rays from the corona heat
the outer disk and induce the corresponding optical brightening; thereafter,
the enhanced accretion rate in the outer disk propagates inwards to the
vicinity of the central black-hole, generating the soft X-rays in the inner
disk region, at a viscous timescale of $\sim 8-12$ days.
