Yes the dwell is to let the spindle start to spin up before Z starts the upfeed. On teh downfeed and at teh end of stroke the X stops and the spindle winds down to a stop this extends teh shaft on teh tap head. So a dwell is needed on the upfeed to allow the spindle to start before Z moves and that retracts the tap head shaft back to neutral. IF the shaft is extended to far on the upfeed at teh top of teh stroke the spring pressure can rip out the first tthread or 2.
(;-) TP