The Malström features two Modulators (mod:A and mod:B) These are in fact another type of oscillators, called LFOs (Low Frequency Oscillators). They each generate a waveform and a frequency, much like osc:A and osc:B. However, there are a couple of important differences:
Mod:A and mod:B do not generate sound. They are instead used for modulating various parameters to change the character of the sound.
They only generate waveforms of low frequency.
Furthermore, both modulators are tempo syncable and possible to use in one shot mode, in which case they will actually work like envelopes.