When a signal has been sent to a process, it is queued in the task structure, and the queue is periodically checked. In this port, the check happens on every kernel mode exit. If a signal needs to be delivered, a SIGUSR2 is sent to the underlying process. The SIGUSR2 handler runs on the process stack, and it executes the process signal handler by simply making a procedure call to it.
Things are a little more complicated if the signal is to be delivered on a different, process-specified, stack. In this case, the alternate stack state, which is composed of register values and stack state, is imposed on the process with ptrace. This new state puts the process in the signal delivery code on the alternate stack, which invokes the process signal handler.