He is talking about the one from ViaBTC[1].
And there is also this one from AntPool[2] which is also free and requires you to register an account.

Honestly, there is no need to pay $4 on a shady paid accelerator. I always send my transactions with a 1~20 sat/byte fee, use these free accelerators and get a confirmation in the same day.

[1] https://pool.viabtc.com/tools/txaccelerator/
[2] https://www.antpool.com/user/prioritiseTransaction.htm?m=savePrioritiseTx