Think of +60k on steroids...
Using a 1 meg simm where the (soft) kernel has access to all pages (60+4).
The kernel (in low mem) can use any high 60k page, load up all nice stuff.
During context switch handled by kernel we do whatever we have to, then we toggle some switch, changing to a user page...
Control circuit is now totally disabled, user programs can't change what pages are mapped into the system.
How do we get back? IRQ!
IRQ automatically switches in the kernel pages to let us do the context switches, switch in user page and lock out page circuit. So no, user processes can't use interrupts, so think of BRK as nice and quick yield() function.
I guess this stuff will come almost for free if I just some day will build the +60k-on-steroids simm.