At the same time, Famitsu Weekly's website (see source link above) has a large batch of screens for the game. They clearly show that the motion-controlled hand movements will be represented in the same way as dance steps, just with a new symbol. The screens also show off the four-player mode, which fills the display with dozens of steps.
The article itself doesn't have too many details on Hottest Party, but Famitsu does mention that the game will include Free Play, Diet Mode (which counts calories based on both dance steps and hand movements), and Group Circuit Play, in which you clear out "stages" which then become available in Free Play.
The article also has a look at the Wii-exclusive dance pad. It has the standard four directional buttons, plus A, B, plus, and minus. It is not yet clear whether the dance pad will be wireless, although the concept art for it does not show a cord.
Thanks to Lord Die Seis for the news tip!