...any idea on how mach synchs the steps i.e if in psuedo code i ask it to go 20 steps cwise x axis then 10 acwise y axis how does mach calculate when the 20 x axis steps are completed is it open loop time based or does it have a count signal from encoder or a completed signal from driver board?
It's open loop. Mach sends out the steps and assumes the machine does what it's told. Partial closing of the loop can be done in the servo drives themselves, but the best you can do is send a servo fault error back into Mach and cause it to stop.
Have a Google around for open vs closed loop. The debates can be tedious, but if you accept that Mach is open loop, you make sure you build the mechanics big and stout enough so this is never a problem.
If you want closed loop, you might have to look at EMC.