it means that you have to stop transmitting for 3.5 times a character, for example if you are communication at 9600bps 8n1, each character takes about 1ms... after you send a data frame, you have to wait at least for 4ms (3.5ms but meh) it doesn't mean that you send 0xFF or 0x00.. you just have to wait for that amount of time... (also remember that when you send a data packet you can't wait over 1.5ms between characters inside the data packet or the slave could restart the packet recieving...)
all of these could be implemented in a slave via timers and interrupts..