how do i jump up without contracting right knee and spin and then kick to get a decap

Trial and error.

To make a hit clean you must make sure you only touch uke with feet and/or hands.

You can A: grab uke and use him to get yourself into the air
B: extendankles and do whatever
C: use your arms to get yourself into the air.

Then after that you can grab or punch his head off with the right tweaking and timing.