Whatever works. Different explanations may click. Without seeing a particular person the only advice I can give is - be relaxed above hips.relaxed does not mean dead or passive.
the 1-2 rocket is a concept to help amateurs to stay sideways and fire in the right sequence without over rotation.... the pros already have this sequence as second nature.
Look at the GIF.
Hips are on the left side. Racket head is on the right side. Kinetic chain is in between - our body and arm. Every dot is a segment. ( We have like 4 segments only)
You apply force only once by hips ( on the left). Pulse propagates through the chain. No force is added anywhere on its way. @FiReFTW pay attention from here )))
Yet every dot on the chart is moving with an acceleration when the pulse comes through ( no force applied anywhere on its way). It tranfers energy from left to right ( hips to racquet.)
Pulse can propagate only through an elastic media. (Elastic means - there is a force that tends to return dots to neutral position ).
It will not propagate through a rigid media.
If you lock a segment ( shoulder, elbow or wrist ) the pulse from hips will stop there. So be elastic , be relaxed all the way from the hips to the wrist.
The trick is that energy transfer comes with a delay. It causes problems with timing. You need to know when to fire hips so that the pulse reaches the wrist by the time the ball comes.
To @Chas Tennis this is described by a the Wave Equation. Acceleration of parts in a wave process does not mean there is always an external force applied ( no need for chest push )