We investigated the X-ray, soft gamma-ray and radio data
of the Galactic black hole binary XTE J1859+226 in the
1999-2000 outburst.
This object showed 5-6 discrete jet ejections within about 30 days around the peak of the outburst,
and thus is suitable to figure out what are the precursors of radio flares and what drives the discrete jet ejections.
We found that jet ejection events occur when the inner radius of the accretion disk (R_in) rapidly approaches the innermost stable circular orbit (ISCO).
Namely, the time derivative of R_in is likely the key parameter to launch jets.
Given the correlations among the X-ray fractional variability amplitude (rms), R_in and the X-ray hardness, previous phenomenological arguments related to jet ejections
("jet line" and sharp drops of rms) seem to be physically
caused by the time variation of R_in.
(I) A rapid shrinkage of R_in down to the ISCO can be a useful index for triggering Target-of-Opportunity observations to catch the moment of the discrete jet production.
(II) When R_in is already at the ISCO, jet ejections are not expected for a while until it leaves the ISCO and gets ready for the next shrinkage of R_in.
(III) It would provide constraints on jet launching mechanisms.
A model which launches jets all the time when R_in is at the ISCO would have difficulties.
We also show how these conditions work in other objects, and discuss towards the "jet forecast".