Assuming that the sine wave is allowed to restart at an arbitrary position, I think the cleanest way might be route the sine wave block through a product block. The second input is 1 if the time is within a "turn on" interval and zero otherwise.
If you need the sine wave to restart from zero on each on/off interval event then it's a little more involved. You'll still need an interval comparison, but that will need to trigger the sine wave to restart its angle. Still use the product block to zero or pass through the sine, but on each new zero crossing, update 
to the time of the latest zero crossing and 
. You can probably use the triggered subsystem construct to accomplish this.
to the time of the latest zero crossing and . You can probably use the triggered subsystem construct to accomplish this.
