The CVL comprises of 2 Constant velocity joints at two ends and a variable length shaft in between.
The varying shaft comprises of two parts, one with external spline on a rod and a female hollow pipe with internal spline.
The external spline rod gets entwined with the female internal splined pipe by sliding inside it and the splines entwined helps to transmit torque.
The splines help to transmit torque and both the parts can slip in and out guided by the splines.
As the tyre travels along a bump up/down the distance between the transmission and tyre changes,the shaft changes its length too.
As distance reduces between tyre and transmission, the external splined rod slips deep inside the female internal splined part and hence the length reduces and vice versa.