Why Frequency Shift Keying sends binary data by shifting the transmitted carrier between two frequencies, one for mark and one for space. In direct FSK the digital data stream is applied to the oscillator itself, pulling a VFO or commanding a DDS/PLL so the RF output jumps between the two frequencies. This is different from AFSK, where audio tones representing the bits are fed into an SSB or FM transmitter and the frequency shift happens indirectly in the modulator.
Watch out Keying an FM transmitter with a sub-audible tone describes an audio-based scheme (AFSK-style), not direct keying of the oscillator, and sub-audible tones are used for CTCSS repeater access rather than data.
Direct FSK = the data wiggles the oscillator. AFSK = audio tones wiggle the modulator.