multi:SetOpt_Pipelining(bitmask)
When enabled, libcurl will attempt to use those protocol features when doing parallel requests to the same hosts.
For pipelining, this means that if you add a second request that can use an already existing connection, the second request will be "piped" on the same connection rather than being executed in parallel.
For multiplexing, this means that follow-up requests can re-use an existing connection and send the new request multiplexed over that at the same time as other transfers are already using that single connection.
There are several other related options that are interesting to tweak and adjust to alter how libcurl spreads out requests on different connections or not etc.
Before 7.43.0, this option was get to 1 and 0 to enable and disable HTTP/1.1 pipelining.
Starting in 7.43.0, bitmask's second bit also has a meaning, and you can ask for pipelining and multiplexing independently of each other by toggling the correct bits.
#CURLPIPE_NOTHING
#CURLPIPE_HTTP1
#CURLPIPE_MULTIPLEX