To multifurcate explicit single branches, you may as well focus the tree-display, move the mouse pointer to the bootstrap value of the branch to eliminate and then press '0' to actually eliminate the branch.
Increasing the length of a branch in MULTIFURC-MODE may cause neighbour branches to become multifurcations. This is irreversible, i.e. reducing the length back to the original value will not "un-multifurcate" the affected branches.
In both modes it is possible to set branchlengths to negative values if expert-mode is enabled.