The first simulation, a cosine wave on the I axis, shows the modulated carrier's amplitude being varied sinusoidally. Perhaps not obvious is the fact that the phase changes 180 degrees each time the baseband-modulating signal crosses zero. Sine on Q axis. The second simulation, a sine wave on the Q axis, shows a similar situation along the Q axis. The counterclockwise rotation means that the modulated signal is at a higher frequency than the unmodulated carrier.
Clockwise rotation means the opposite.
The frequency offset is shown in the spectrum display. In the case of QPSK, the I and Q inputs are momentary dc values. Note that occasionally the phase shift is 180 degrees. To accommodate this situation, the transmitter output power amplifier must be linear over a wide dynamic range to prevent spectral splatter and therefore interference with adjacent frequency channels. That is, a second set of four possible phase states is utilized.