The Cassette Relay (If connected) will Pause the cassette recorder automatically. This was important for multi-load cassette games, where once the first section was loaded in, the cassette could be paused by the computer until more data was required.
This device (As shown on the Hardware page) looks like an integrated curcuit (Chip) but is in fact a reed relay. Read relays require very little power to operate whilst completely isolating the switch mechanism from the actuating side.
The actuating of the Relay is controlled by Bit 6 in Port B of the 6522 VIA (Oric location #0300).
The two contacts appear on the Cassette port, pins 6 and 7. It doesn't matter which way round the contacts are connected to the external Cassette recorder, but care must be taken to keep both voltage and current levels within reasonable ratings.