It will likely run higher than 2.4 GHz in bursts and then throttle itself as heat builds up and/or all cores are busy constantly at which time you should expect full load 1.1 GHz or better.
So - it will run at both speeds and many others.
There is no indication that the Intel CPU works any differently in the MacBook than all the rest recent Mac hardware. The lower number is a steady state / worst case / all cores are active and running and the heat being generated by the CPU is being removed as designed - you get 1.1 GHz clock rate for hours long calculations.
When you wake the Mac from sleeping, it will generally start in power savings mode and the CPU only spins up cores / clock rate when there is work to do, but it will ramp up to the burst GHz (or possibly higher) once you have one thread worth of work to get done.
If your work is single threaded, you can expect performance similar to a 2.4 GHz clock rate CPU unless/until the internal temperature near the CPU raises close to the design limit.
Since this product isn't shipping, only Apple's engineers really know how the case without a blower will cool the processor and what loads will run at which speeds.
No Apple software allows you to "boost" things on previous MacBook / Pro / Air - so it's hard to guess if they will continue that pattern.