Torque converter

think of a converter as one fan blowing air at another .At some point it will move enough air to turn the other fan and this would be the stall speed.If you change the pitch of the fan blades in either fan the speed that the second fan starts turning at will change.
...Now rev up the blowing fan quickly and it will spin a bit faster before the second fan starts to turn because it will slip briefly until the air starts turning the other fan...this would be the flash stall speed.
In a transmission the prime mover would be oil rather than air...