.. be aware, then, that IAS values are very dependent on PEC which is very, very dependent on flow variations in the vicinity of the static source, especially for keel mounted ports. Most of us are well aware of the small Cessna phenomenon of pulling the aircraft back well into a stall and then watching the IAS disappear off the clock. This doesn't indicate that the speed is zero, only that the instrument is telling porkies ....
The alternate static source in the cockpit is for emergency use in the event of water, ice or other contamination of the the external static source or the plumbing to that source.