  Mechanism of di-pion transitions $nS\to n'S\pi\pi(n=3,2; n'=2,1)$ in
bottomonium and charmonium is studied with the use of the chiral
string-breaking Lagrangian allowing for the emission of any number of
$\pi(K,\eta),$ and not containing fitting parameters. The transition amplitude
contains two terms, $M=a-b$, where first term (a) refers to subsequent one-pion
emission: $\Upsilon(nS)\to\pi B\bar B^*\to\pi\Upsilon(n'S)\pi$ and second term
(b) refers to two-pion emission: $\Upsilon(nS)\to\pi\pi B\bar
B\to\pi\pi\Upsilon(n'S)$. The one-parameter formula for the di-pion mass
distribution is derived, $\frac{dw}{dq}\sim$(phase space) $|\eta-x|^2$, where
$x=\frac{q^2-4m^2_\pi}{q^2_{max}-4m^2_\pi},$ $q^2\equiv M^2_{\pi\pi}$. The
parameter $\eta$ dependent on the process is calculated, using SHO wave
functions and imposing PCAC restrictions (Adler zero) on amplitudes a,b. The
resulting di-pion mass distributions are in agreement with experimental data.
