Role of Hg2+ as catalyst in the reaction of addition of hydrogen halides to alkynes $\ce{Hg^2+}$ can be used as a catalyst. But what is its role? How does it increase the rate of reaction.

I think that the vacant orbitals in Hg would help in the process of ionisation of hydrogen halides. Is this concept correct? please help
The main use of $\ce{Hg}$$^{\text{2+}}$ is to polarize the electron cloud of the alkyne.
Else the attacking specie wont be able to approach the bond