Hello Francesco,
FlexStack is the particular stack type supported on C2960-S switches.
So I would expect the cable format and the plug format to be specific of this type of switches.
http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps6406/qa_c67-577519.html
See the FlexStack section
>> Also, FlexStack cables have different connectors than the StackWise cables. (those of C3750)
Q. What is Cisco FlexStack stacking?
A. Cisco FlexStack is a hot-swappable, modular stacking solution that provides a true stacking capability, with all switches acting as a single switching unit with a unified data plane, using a single IP address. Cisco FlexStack allows the stacking of up to four switches, with a throughput of 20 gigabits per second.

Stacking is supported only on Catalyst 2960-S switches running the LAN base image.
All member devices have to be C2960-S switches. The number of ports on each member switch is not relevant (this happens already for other stacks like C3750).
About the way to interconnect switch members with Flexstack cable you can use as a reference the figures 7-1 and 7-2 from the following Chapter on managing stacks from Configuration Guide of C2960 12.2 (58)SE
http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_58_se/configuration/guide/swstack.html#wp1142379
The cables have to form a dual ring so in the case of two member stacks the ports are closed between the two devices.
Hope to help
Giuseppe