If we look again at the conservation of energy equation,
v2t+v2s=c2
what happens when vs>c,
v2t=−V2 where V2=v2s−c2
v′t=iV
↖+↗+↙=⇈
vt vs iv′t c
A complex component develops in vt that cancels that part of vs that is greater that c. The use of iv′t is to illustrate that vt and iv′t are not of the same magnitude.
Similarly, if we develop a complex component in vs we will then be able to cancel part of vt and given a great enough complex component in vs, we will be able to make vt negative. vt, vs are dummy variables we can swap them and we have
↖+↗+↙=⇈
vs vt iv′s c
Remember that c is a constant. If we are able to generate iv′s we can then slow vt and even reverse vt.